[test-suite] Support GCC compilation of Microbenchmark MemFunctions.

[naromero77]

Summary

GCC and Clang diverged in how attribute is supported. GCC gives a compile-time error if there is an attribute in the function-definition and needs to be in the forward declaration. Clang does not like attribute in the forward declaration and requires it in the function-definition. This is the only benchmark in the test suite not compiling with GCC.

[courbet]

Thanks for the patch. Apparently both gcc and clang are fine if the attribute is placed before the definition: https://godbolt.org/z/5aTrcb, so we could do without the ifdefs.
(Though gcc does not know about the attribute, but that's also the case with the current version)

[naromero77]

OK, let's do that instead. Best to avoid ifdefs. Will update patch soon.

[naromero77]

• Place attribute before the definition and avoid ifdef.

[naromero77]

Committed as https://github.com/llvm/llvm-test-suite/commit/7c17aa10cd598129896b9501b73cb583c8bcdeb5

MicroBenchmarks/MemFunctions/main.cpp

 #include <cstring>
 #include <iostream>
 #include <vector>
 
 #include "benchmark/benchmark.h"
 
 // This function prevents the compiler from interfering with `memcmp` and
 // makes sure the function is called.
-int RealMemCmp(const char *p, const char *q, size_t s)
-    __attribute__((no_builtin("memcmp"))) {
+__attribute__((no_builtin("memcmp")))
+int RealMemCmp(const char *p, const char *q, size_t s) {
   return memcmp(p, q, s);
 }
 
 // Benchmarks `memcmp(p, q, size) OP 0` where n is known at compile time and OP
 // is defined by `Pred`. The compiler typically inlines the memcmp + comparion
 // to loads and compares.
 template <int kSize, typename Pred, typename Mod>
 void BM_MemCmp(benchmark::State& state) {