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.
When compiling Flang-RT with Clang, query Clang for the GCC installation
it uses. If found, create `quadmath_wrapper.h` that points to the
`quadmath.h` of that GCC installation.
`quadmath.h` is only available when compiling with gcc, and Clang has no
equivalent even though gcc's version compiles fine with Clang (at least
up to and including gcc 13). It is still available into gcc's
installation resource dir (in constrast to a system-wide indirectory
such as `/usr/include` or `/usr/local/include`) and therefore not
available to any compiler other than the gcc of that installation.
quadmath may also be a different OS package than gcc itself, so it is
not necessarily presesent.
Clang actually already appropriates a GCC installation for its libraries
such that `libquadmath.a` is already found, but it does not do so for
the include paths. Because adding that directory to the header search
path may have wide-reaching consquences, we create only a wrapper header
that points to the real `quadmath.h` in the same GCC installation that
Clang uses.
