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.
9b8215ce9d