Allow all supportable non-type attributes to be used with #pragma clang attribute.

Summary:
We previously disallowed use of undocumented attributes with #pragma clang
attribute, but the justification for doing so was weak and it prevented many
reasonable use cases.

Reviewers: aaron.ballman, arphaman

Subscribers: cfe-commits, rnk, benlangmuir, dexonsmith, erik.pilkington

Differential Revision: https://reviews.llvm.org/D51507

llvm-svn: 341437