MdePkg: Fix null macros for XCODE5 and CLANG

When building OvmfPkg in RELEASE mode in the XCODE5 toolchain, the
ASSERT_EFI_ERROR change prevents this error:

.../MdePkg/Library/UefiMemoryAllocationLib/MemoryAllocationLib.c:141:15:
error: variable 'Status' set but not used [-Werror,-Wunused-but-set-variable]
  EFI_STATUS  Status;
              ^

which is currently stopping the build.

When building in RELEASE mode in the CLANGPDB toolchain,the DEBUG macro
change prevents this error:

.../edk2/OvmfPkg/VirtioSerialDxe/VirtioSerial.c:28:22: error:
variable 'EventNames' is not needed and will not be
emitted [-Werror,-Wunneeded-internal-declaration]
STATIC CONST CHAR8  *EventNames[] = {
                     ^

which is currently stopping the build.

CLANGDWARF produces the same error as CLANGPDB above, if
-Wno-unneeded-internal-declaration is removed from its build flags.
With the null DEBUG macro change, this warning suppression
can be removed from CLANGDWARF, which is considered a benefit
as it has the potential to catch real coding errors. This is
done in a subsequent commit.

This commit has the desirable side effect that we no longer require
(and cannot use) explicit `#ifndef MDEPKG_NDEBUG` around items only
used in DEBUG macros. This requires the ArmPkg change made here to
be in the same commit as the MdePkg changes.

Note: In common with existing macros in EDK II, including the pre-existing
and unchanged DEBUG/NOOPT versions of the macros which are modified here,
we use the standard approach of adding `do { ... } while (FALSE)` wrapping
to ensure that the macros behave correctly with surrounding code
(e.g. require a following ';' and do not combine in unexpected ways with
nearby conditionals).

Continuous-integration-options: PatchCheck.ignore-multi-package
Co-authored-by: Mikhail Krichanov <krichanov@ispras.ru>
Signed-off-by: Mike Beaton <mjsbeaton@gmail.com>

ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c

@@ -157,7 +157,6 @@ DescribeExceptionSyndrome (
   DEBUG ((DEBUG_ERROR, "\n %a \n", Message));
 }
 
-#ifndef MDEPKG_NDEBUG
 STATIC
 CONST CHAR8 *
 BaseName (
@@ -177,8 +176,6 @@ BaseName (
   return Str;
 }
 
-#endif
-
 /**
   This is the default action to take on an unexpected exception
 

MdePkg/Include/Library/DebugLib.h

@@ -8,6 +8,13 @@
   of size reduction when compiler optimization is disabled. If MDEPKG_NDEBUG is
   defined, then debug and assert related macros wrapped by it are the NULL implementations.
 
+  The implementations of the macros used when MDEPKG_NDEBUG is defined rely on the fact that
+  directly unreachable code is pruned, even with compiler optimization disabled (which has
+  been confirmed by generated code size tests on supported compilers). The advantage of
+  implementations which consume their arguments within directly unreachable code is that
+  compilers understand this, and stop warning about variables which would become unused when
+  MDEPKG_NDEBUG is defined if the macros had completely empty definitions.
+
 Copyright (c) 2006 - 2020, Intel Corporation. All rights reserved.<BR>
 SPDX-License-Identifier: BSD-2-Clause-Patent
 
@@ -403,7 +410,12 @@ UnitTestDebugAssert (
       }                             \
     } while (FALSE)
 #else
-#define ASSERT(Expression)
+#define ASSERT(Expression)       \
+    do {                           \
+      if (FALSE) {                 \
+        (VOID) (Expression);       \
+      }                            \
+    } while (FALSE)
 #endif
 
 /**
@@ -426,7 +438,12 @@ UnitTestDebugAssert (
       }                            \
     } while (FALSE)
 #else
-#define DEBUG(Expression)
+#define DEBUG(Expression)        \
+    do {                           \
+      if (FALSE) {                 \
+        _DEBUGLIB_DEBUG (Expression);       \
+      }                            \
+    } while (FALSE)
 #endif
 
 /**
@@ -452,7 +469,12 @@ UnitTestDebugAssert (
       }                                                                                  \
     } while (FALSE)
 #else
-#define ASSERT_EFI_ERROR(StatusParameter)
+#define ASSERT_EFI_ERROR(StatusParameter)                                             \
+    do {                                                                                \
+      if (FALSE) {                                                                      \
+        (VOID) (StatusParameter);                                                       \
+      }                                                                                 \
+    } while (FALSE)
 #endif
 
 /**
@@ -479,7 +501,12 @@ UnitTestDebugAssert (
       }                                                                 \
     } while (FALSE)
 #else
-#define ASSERT_RETURN_ERROR(StatusParameter)
+#define ASSERT_RETURN_ERROR(StatusParameter)                          \
+    do {                                                                \
+      if (FALSE) {                                                      \
+        (VOID) (StatusParameter);                                       \
+      }                                                                 \
+    } while (FALSE)
 #endif
 
 /**