gdb/fortran: Allow for using Flang in Fortran testing

In lib/fortran.exp, in the helper function fortran_int4, there is
currently no support for the LLVM Fortran compiler, Flang.  As a
result we return the default pattern 'unknown' to match against all
4-byte integer types, which causes many tests to fail.

The same is true for all of the other helper functions related to
finding a suitable type pattern.

This commit adds support for Flang.  There should be no change when
testing with gfortran.

gdb/testsuite/ChangeLog:

	* lib/fortran.exp (fortran_int4): Handle clang.
	(fortran_int8): Likewise.
	(fortran_real4): Likewise.
	(fortran_real8): Likewise.
	(fortran_complex4): Likewise.
	(fortran_logical4): Likewise.
	(fortran_character1): Likewise.

Change-Id: Ife0d9828f78361fbd992bf21af746042b017dafc