Initial support for reading type information from PDBs.

This implements a PDBASTParser and corresponding logic in
SymbolFilePDB to do type lookup by name.  This is just a first
pass and leaves many aspects of type lookup unimplemented, and
just focuses on laying the framework.  With this patch, you should
be able to lookup basic types by name from a PDB.

Full class definitions are not completed yet, we will instead
just return a forward declaration of the class.

Differential Revision: http://reviews.llvm.org/D18848
Reviewed by: Greg Clayton

llvm-svn: 266392