[Driver] -p: change from unused warning to error for most targets

AIX and OpenBSD seem to use -p. For most targets (at least FreeBSD and Linux),
-p is legacy (GCC freebsd has a warning). We don't want the uses to grow, so
making -p an alias for -pg is not recommended. I think the uses are small.

Reviewed By: mgorny

Differential Revision: https://reviews.llvm.org/D138255