Convert FALLTHROUGH comments to C++17 [[fallthrough]] attribute

Use conditionals and a macro to avoid provoking compilers that do
not support this attribute.

Adds a macro named CPP_ATTRIBUTE_FALLTHROUGH which is invoked right
before a following case statement to declare that the intent is to
fall through.  Example:

    ...
    case xxx:
       ...
       CPP_ATTRIBUTE_FALLTHROUGH;
    case yyy:
    ...

The gcc/clang alternative of adding comments that contain "FALLTHROUGH"
suffers from the problem that *by default* ccache strips the comments
so that they are not present for the real compilation.

Change-Id: I77ddeb7dae46db8398b014a93f6a71bedc64ada9
Signed-off-by: Dale Stimson <dale.b.stimson@intel.com>

common/compiler_support.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2018, Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included
+ * in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+ * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#pragma once
+
+#if !defined __has_cpp_attribute
+
+#define CPP_ATTRIBUTE_FALLTHROUGH
+
+#else
+
+#if !__has_cpp_attribute(fallthrough)
+#define CPP_ATTRIBUTE_FALLTHROUGH
+#else
+#define CPP_ATTRIBUTE_FALLTHROUGH [[fallthrough]]
+#endif
+
+#endif