[CMake] Disable -Werror for CMake checks

-Werror may cause some of the CMake checks to fail, so we disable
it even if it's enabled for the build itself.

Differential Revision: https://reviews.llvm.org/D35924

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309235 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/modules/HandleLLVMOptions.cmake

@@ -381,6 +381,7 @@ if( MSVC )
 
 elseif( LLVM_COMPILER_IS_GCC_COMPATIBLE )
   append_if(LLVM_ENABLE_WERROR "-Werror" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+  append_if(LLVM_ENABLE_WERROR "-Wno-error" CMAKE_REQUIRED_FLAGS)
   add_flag_if_supported("-Werror=date-time" WERROR_DATE_TIME)
   if (LLVM_ENABLE_CXX1Y)
     check_cxx_compiler_flag("-std=c++1y" CXX_SUPPORTS_CXX1Y)