Handle _Pragma on a u8, u, or U string literal per the C11 specification. Also
handle raw string literals here. C++11 doesn't yet specify how they will
behave, but discussion on core suggests that we should just strip off
everything but the r-char-sequence.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176779 91177308-0d34-0410-b5e6-96231b3b80d8