Implement basic support for parsing #pragma comment, a microsoft extension

Implement basic support for parsing #pragma comment, a microsoft extension
documented here:
http://msdn.microsoft.com/en-us/library/7f0aews7(VS.80).aspx

This is according to my understanding reading the docs, I don't know if it
really agrees fully with what VC++ allows.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@62317 91177308-0d34-0410-b5e6-96231b3b80d8