Lex and ignore Microsoft's #pragma warning(...)

Summary:
This fixes PR17145 and avoids unknown pragma warnings.

This change does not attempt to map MSVC warning numbers to clang
warning flags.  Perhaps in the future we will implement a mapping for
some common subset of Microsoft warnings, but for now we don't.

Reviewers: rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1652