[libclang] Compute the right spelling location (#72400)

Locations inside macro expansions have different spelling/expansion
locations. Apply a FIXME to make the libclang function
clang_getSpellingLocation return the right spelling location, and adapt
the testsuite driver code to use the file location rather than the
spelling location to compute source ranges.

Co-authored-by: Matthieu Eyraud <eyraud@adacore.com>