CMAKE: apply -O3 for mingw clang

Differential Revision: https://reviews.llvm.org/D41596

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324570 91177308-0d34-0410-b5e6-96231b3b80d8

Author: martell

CMakeLists.txt

@@ -834,7 +834,7 @@ endif()
 include(AddLLVM)
 include(TableGen)
 
-if( MINGW )
+if( MINGW AND NOT "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang" )
   # People report that -O3 is unreliable on MinGW. The traditional
   # build also uses -O2 for that reason:
   llvm_replace_compiler_option(CMAKE_CXX_FLAGS_RELEASE "-O3" "-O2")