[cmake] Remove duplicate command line options from build

CMAKE_CXX_FLAGS was added twice to the command line. This causes the command 
line options to be doubled which works until it doesn't as not all options 
can be specified twice. 

For example,

clang-cl foo.c /GS- /GS- -mllvm -small-loop-cost=1 -mllvm -small-loop-cost=1
clang (LLVM option parsing): for the -small-loop-cost option: may only occur zero or one times!

llvm-svn: 329817

compiler-rt/cmake/Modules/AddCompilerRT.cmake

@@ -63,8 +63,8 @@ function(add_compiler_rt_object_libraries name)
       list(REMOVE_ITEM target_flags "-msse3")
     endif()
 
-    set_target_compile_flags(${libname}
-      ${CMAKE_CXX_FLAGS} ${extra_cflags_${libname}} ${target_flags})
+    set_target_compile_flags(${libname} ${extra_cflags_${libname}} ${target_flags})
+
     set_property(TARGET ${libname} APPEND PROPERTY
       COMPILE_DEFINITIONS ${LIB_DEFS})
     set_target_properties(${libname} PROPERTIES FOLDER "Compiler-RT Libraries")