[benchmark] Silence warning by applying upstream patch

[kbobyrev]

Summary

Compiling benchmark library (introduced in D50894) with the latest bootstrapped Clang produces a lot of warnings, this issue was addressed in the upstream patch I pushed earlier.

Upstream patch: https://github.com/google/benchmark/commit/f85304e4e3a0e4e1bf15b91720df4a19e90b589f

README.LLVM notes were updated to reflect the latest changes.

[lebedev.ri]

LG to unbreak the build (this should have been in the original differential in the first place.)

[kbobyrev]

In D51342#1215510, @lebedev.ri wrote:

LG to unbreak the build (this should have been in the original differential in the first place.)

Oh, sorry, I deliberately left it out because I thought I should split diffs into smaller chunks to make the review process easier.

llvm/utils/benchmark/README.LLVM

 LLVM notes
 ----------
 
-This directory contains the Google Benchmark source code. This directory is
-under a different license than LLVM.
+This directory contains the Google Benchmark source code. Currently, the checked
+Benchmark library version is v1.4.1.
+
+This directory is under a different license than LLVM.
 
 Changes:
 
 * Bazel BUILD files are removed from the library
+* https://github.com/google/benchmark/commit/f85304e4e3a0e4e1bf15b91720df4a19e90b589f
+  is applied on top of the v1.4.1 to silence compiler warnings

llvm/utils/benchmark/include/benchmark/benchmark.h

 #define BENCHMARK_DEPRECATED_MSG(msg)
 #define BENCHMARK_WARNING_MSG(msg) __pragma(message(__FILE__ "(" BENCHMARK_INTERNAL_TOSTRING(__LINE__) ") : warning note: " msg))
 #endif
 
 #if defined(__GNUC__) && !defined(__clang__)
 #define BENCHMARK_GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__)
 #endif
 
+#ifndef __has_builtin
+#define __has_builtin(x) 0
+#endif
+
+#if defined(__GNUC__) || __has_builtin(__builtin_unreachable)
+  #define BENCHMARK_UNREACHABLE() __builtin_unreachable()
+#elif defined(_MSC_VER)
+  #define BENCHMARK_UNREACHABLE() __assume(false)
+#else
+  #define BENCHMARK_UNREACHABLE() ((void)0)
+#endif
+
 namespace benchmark {
 class BenchmarkReporter;
 
 void Initialize(int* argc, char** argv);
 
 // Report to stdout all arguments in 'argv' as unrecognized except the first.
 // Returns true there is at least on unrecognized argument (i.e. 'argc' > 1).
 bool ReportUnrecognizedArguments(int argc, char** argv);

 
 inline const char* GetTimeUnitString(TimeUnit unit) {
   switch (unit) {
     case kMillisecond:
       return "ms";
     case kMicrosecond:
       return "us";
     case kNanosecond:
-    default:
       return "ns";
   }
+  BENCHMARK_UNREACHABLE();
 }
 
 inline double GetTimeUnitMultiplier(TimeUnit unit) {
   switch (unit) {
     case kMillisecond:
       return 1e3;
     case kMicrosecond:
       return 1e6;
     case kNanosecond:
-    default:
       return 1e9;
   }
+  BENCHMARK_UNREACHABLE();
 }
 
 } // namespace benchmark
 
 #endif  // BENCHMARK_BENCHMARK_H_

llvm/utils/benchmark/src/internal_macros.h

 #ifndef BENCHMARK_INTERNAL_MACROS_H_
 #define BENCHMARK_INTERNAL_MACROS_H_
 
 #include "benchmark/benchmark.h"
 
 #ifndef __has_feature
 #define __has_feature(x) 0
 #endif
-#ifndef __has_builtin
-#define __has_builtin(x) 0
-#endif
 
 #if defined(__clang__)
   #if !defined(COMPILER_CLANG)
     #define COMPILER_CLANG
   #endif
 #elif defined(_MSC_VER)
   #if !defined(COMPILER_MSVC)
     #define COMPILER_MSVC

 #endif
 
 #if defined(COMPILER_CLANG) || defined(COMPILER_GCC)
   #define BENCHMARK_MAYBE_UNUSED __attribute__((unused))
 #else
   #define BENCHMARK_MAYBE_UNUSED
 #endif
 
-#if defined(COMPILER_GCC) || __has_builtin(__builtin_unreachable)
-  #define BENCHMARK_UNREACHABLE() __builtin_unreachable()
-#elif defined(COMPILER_MSVC)
-  #define BENCHMARK_UNREACHABLE() __assume(false)
-#else
-  #define BENCHMARK_UNREACHABLE() ((void)0)
-#endif
-
 #endif  // BENCHMARK_INTERNAL_MACROS_H_