[compiler-rt] Pass CMake C and C++ flags to the custom libc++

[phosek]

Summary

When building the custom libc++ that's used by libFuzzer as well as
MSan and TSan tests, passthrough the C and C++ flags that were passed
to the compiler-rt CMake build. These may be needed to successfuly
compile the library on a particular platform.

[phosek]

I think this addresses the issue that D49397 was trying to solve.

[phosek]

Ping?

[morehouse]

LGTM

compiler-rt/cmake/Modules/AddCompilerRT.cmake

set(PASSTHROUGH_VARIABLES
     CMAKE_SYSROOT
     CMAKE_SYSTEM_NAME)
   foreach(variable ${PASSTHROUGH_VARIABLES})
     if(${variable})
       list(APPEND CMAKE_PASSTHROUGH_VARIABLES -D${variable}=${${variable}})
     endif()
   endforeach()
 
-  string(REPLACE ";" " " FLAGS_STRING "${LIBCXX_CFLAGS}")
-  set(LIBCXX_C_FLAGS "${FLAGS_STRING}")
-  set(LIBCXX_CXX_FLAGS "${FLAGS_STRING}")
+  get_property(C_FLAGS CACHE CMAKE_C_FLAGS PROPERTY VALUE)
+  string(REPLACE ";" " " LIBCXX_C_FLAGS "${LIBCXX_CFLAGS}")
+  set(LIBCXX_C_FLAGS "${LIBCXX_C_FLAGS} ${C_FLAGS}")
+  get_property(CXX_FLAGS CACHE CMAKE_CXX_FLAGS PROPERTY VALUE)
+  string(REPLACE ";" " " LIBCXX_CXX_FLAGS "${LIBCXX_CFLAGS}")
+  set(LIBCXX_CXX_FLAGS "${LIBCXX_CXX_FLAGS} ${CXX_FLAGS}")
 
   ExternalProject_Add(${name}
     DEPENDS ${name}-clobber ${LIBCXX_DEPS}
     PREFIX ${prefix}
     SOURCE_DIR ${COMPILER_RT_LIBCXX_PATH}
     STAMP_DIR ${STAMP_DIR}
     BINARY_DIR ${BINARY_DIR}
     CMAKE_ARGS ${CMAKE_PASSTHROUGH_VARIABLES}