[SampleProfile] Fix UB in Demangler invocation. (#137659)

Currently the backing buffer of a `std::vector<char>` is passed[1] to
`Demangler.getFunctionBaseName`. However, deeply inside the call stack
`OutputBuffer::grow` will call[2] `std::realloc` if it needs to grow the
buffer, leading to UB.

The demangler APIs specify[3] that "`Buf` and `N` behave like the second
and third parameters to `__cxa_demangle`" and the docs for the latter
say[4] that the output buffer must be allocated with `malloc` (but can
also be `NULL` and will then be realloced accordingly).

Note: PR #135863 changed this from a stack array to a `std::vector` and
increased the size to 65K, but this can still lead to a crash if the
demangled name is longer than that - yes, I'm surprised that a >65K-long
function name happens in practice...

[1]:
d7e631c7cd/llvm/lib/Transforms/IPO/SampleProfileMatcher.cpp (L744)
[2]:
d7e631c7cd/llvm/include/llvm/Demangle/Utility.h (L50)
[3]:
d7e631c7cd/llvm/include/llvm/Demangle/Demangle.h (L92-L93)
[4]:
https://gcc.gnu.org/onlinedocs/libstdc++/libstdc++-html-USERS-4.3/a01696.html

llvm/lib/Transforms/IPO/SampleProfileMatcher.cpp

@@ -737,14 +737,16 @@ bool SampleProfileMatcher::functionMatchesProfileHelper(
     auto FunctionName = FName.str();
     if (Demangler.partialDemangle(FunctionName.c_str()))
       return std::string();
-    constexpr size_t MaxBaseNameSize = 65536;
-    std::vector<char> BaseNameBuf(MaxBaseNameSize, 0);
-    size_t BaseNameSize = MaxBaseNameSize;
-    char *BaseNamePtr =
-        Demangler.getFunctionBaseName(BaseNameBuf.data(), &BaseNameSize);
-    return (BaseNamePtr && BaseNameSize)
-               ? std::string(BaseNamePtr, BaseNameSize)
-               : std::string();
+    size_t BaseNameSize = 0;
+    // The demangler API follows the __cxa_demangle one, and thus needs a
+    // pointer that originates from malloc (or nullptr) and the caller is
+    // responsible for free()-ing the buffer.
+    char *BaseNamePtr = Demangler.getFunctionBaseName(nullptr, &BaseNameSize);
+    std::string Result = (BaseNamePtr && BaseNameSize)
+                             ? std::string(BaseNamePtr, BaseNameSize)
+                             : std::string();
+    free(BaseNamePtr);
+    return Result;
   };
   auto IRBaseName = GetBaseName(IRFunc.getName());
   auto ProfBaseName = GetBaseName(ProfFunc.stringRef());