Fix a couple of issues with literal-operator-id parsing, and provide recovery

for a few kinds of error. Specifically:

Since we're after translation phase 6, the "" token might be formed by multiple
source-level string literals. Checking the token width is not a correct way of
detecting empty string literals, due to escaped newlines. Diagnose and recover
from a missing space between "" and suffix, and from string literals other than
"", which are followed by a suffix.

llvm-svn: 152348