Fix a nasty bug where inside StringLiteralParser:

1. We would assume that the length of the string literal token was at least 2
2. We would allocate a buffer with size length-2

And when the stars aligned (one of which would be an invalid source location due to stale PCH)
The length would be 0 and we would try to allocate a 4GB buffer.

Add checks for this corner case and a bunch of asserts.
(We really really should have had an assert for 1.).

Note that there's no test case since I couldn't get one (it was major PITA to reproduce),
maybe later.

llvm-svn: 131492