When building Flang with Clang, we need to do the same quadmath.h
wrapping as we do for flang-rt. I extracted the CMake code
into FlangCommon.cmake, and cleaned up the arguments passing
to execute_process (note that `-###` was treated as `-` in the original
code, because `#` starts a comment). I believe the Clang command
does not require the input source file, so I removed it as well.
