[flang] Check there's no dependency on C++ libs. NFC

Add a test to make sure the flang runtime doesn't pull in the C++
runtime libraries.

This is achieved by adding a C file that calls some functions from the
runtime (currently only CpuTime, but we should probably add anything
complicated enough, e.g. IO-related things). We force the C compiler to
use -std=c90 to make sure it's really in C mode (we don't really care
which version of the standard, this one is probably more widely
available). We only enable this test if CMAKE_C_COMPILER is set to
something (which is probably always true in practice).

This is a recommit of 7ddbf263, with 2 fixes:
* Replace C++ comments with C comments
* Only enable the test if libFortranRuntime.a exists (this might not be
the case if e.g. BUILD_SHARED_LIBS=On)

Differential Revision: https://reviews.llvm.org/D104290