Ignore pgcc-llvm and friends, default to pgcc (#11703)

This change reverts to the previous behavior of only looking for pgcc
and friends, not pgcc-llvm and friends.

The llvm variant doesn't support all the same features as the
traditional variant of the pgi code generator; this change avoids
treating the llvm variant as a default pgi compiler.

This retains the changes in #10704 which accept the "LLVM" suffix of
the version string of the PGI compiler, which allows users to
explicitly add the llvm-pgi compiler if desired.

lib/spack/spack/compilers/pgi.py

@@ -20,10 +20,6 @@ class Pgi(Compiler):
     # Subclasses use possible names of Fortran 90 compiler
     fc_names = ['pgfortran', 'pgf95', 'pgf90']
 
-    # LLVM-enabled PGI compilers add a '-llvm' suffix:
-    #   pgcc-llvm, pgc++llvm, pgfortran-llvm
-    suffixes = ['-?llvm']
-
     # Named wrapper links within build_env_path
     link_paths = {'cc': 'pgi/pgcc',
                   'cxx': 'pgi/pgc++',