[llvm][CMake] Check dependency cxx source compiles (#68549)

If a CMake project doesn't enable the C language, then the CMake FFI and
Terminfo find modules will fail their checks for compilation and
linking.

This commit allows projects to enable only C++ by first checking if a C
compiler is set before testing C source compilation; if not, it checks
whether C++ compilation succeeds.

Fixes #53950