Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -829,7 +829,7 @@
 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")