[llvm-test-suite][Fortran/gfortran] Categorize compile-only discrepancies in Fortran/gfortran/regression

Take a pass through the compilation-only tests in the GNU Fortran regression
test suite that either compile when they shouldn't or don't compile when they
should.  Categorize the discrepancies according to root causes for each
test, where it's obvious.

Differential Revision: https://reviews.llvm.org/D154866

Author: klausler