Add #pragma clang attribute

The new '#pragma clang attribute' directive can be used to apply attributes to
multiple declarations. An attribute must satisfy the following conditions to
be supported by the pragma:
- It must have a subject list that's defined in the TableGen file.
- It must be documented.
- It must not be late parsed.
- It must have a GNU/C++11 spelling.

Differential Revision: https://reviews.llvm.org/D30009


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300539 91177308-0d34-0410-b5e6-96231b3b80d8