[benchmarks] Move libcxx's fork of google/benchmark and llvm/utils'

under third-party

This change:
- moves the libcxx copy of `google/benchmark` to
`third-party/benchmkark`
- points the 2 uses of the library (libcxx and llvm/utils) to this copy

We picked the licxx copy because it is the most up to date.

Differential Revision: https://reviews.llvm.org/D112012

Author: mtrofin

libcxx/benchmarks/CMakeLists.txt

@@ -41,7 +41,7 @@ ExternalProject_Add(google-benchmark-libcxx
         EXCLUDE_FROM_ALL ON
         DEPENDS cxx cxx-headers
         PREFIX benchmark-libcxx
-        SOURCE_DIR ${LIBCXX_SOURCE_DIR}/utils/google-benchmark
+	SOURCE_DIR ${LLVM_THIRD_PARTY_DIR}/benchmark
         INSTALL_DIR ${CMAKE_CURRENT_BINARY_DIR}/benchmark-libcxx
         CMAKE_CACHE_ARGS
           -DCMAKE_C_COMPILER:STRING=${CMAKE_C_COMPILER}
@@ -66,7 +66,7 @@ if (LIBCXX_BENCHMARK_NATIVE_STDLIB)
   ExternalProject_Add(google-benchmark-native
         EXCLUDE_FROM_ALL ON
         PREFIX benchmark-native
-        SOURCE_DIR ${LIBCXX_SOURCE_DIR}/utils/google-benchmark
+	SOURCE_DIR ${LLVM_THIRD_PARTY_DIR}/benchmark
         INSTALL_DIR ${CMAKE_CURRENT_BINARY_DIR}/benchmark-native
         CMAKE_CACHE_ARGS
           -DCMAKE_C_COMPILER:STRING=${CMAKE_C_COMPILER}