PR17359: Fix off-by-one OOB on _Pragma("") and an unescaping bug

Previously the code would reduce a run of backslashes to a single
backslash, and now it will properly leave behind every other backslash.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191382 91177308-0d34-0410-b5e6-96231b3b80d8