Improved error recovery for _Pragma

Summary:
Currently, if the argument to _Pragma is not a parenthesised string
literal, the bad token will be consumed, as well as the ')', if present.
If additional bad tokens are passed to the _Pragma, this results in
extra error messages which may distract from the true problem.

The proposed patch causes all tokens to be consumed until the closing
')' or a new line, whichever is reached first.

Reviewers: hfinkel, rsmith

Subscribers: hubert.reinterpretcast, fraggamuffin, rnk, cfe-commits

Differential Revision: http://reviews.llvm.org/D8308

Patch by Rachel Craik!

llvm-svn: 243692