Disable _Pragma during HTML macro rewriting to keep from crashing.

The preprocessor's handling of diagnostic push/pops is stateful, so
encountering pragmas during a re-parse causes problems. HTMLRewrite
already filters out normal # directives including #pragma, so it's
clear it's not expected to be interpreting pragmas in this mode.

This fix adds a flag to Preprocessor to explicitly disable pragmas.
The "right" fix might be to separate pragma lexing from pragma
parsing so that we can throw away pragmas like we do preprocessor
directives, but right now it's important to get the fix in.

Note that this has nothing to do with the "hack" of re-using the
input preprocessor in HTMLRewrite. Even if we someday copy the
preprocessor instead of re-using it, the copy would (and should) include
the diagnostic level tables and have the same problems.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158214 91177308-0d34-0410-b5e6-96231b3b80d8