[PDB] Enable NativeSession to create symbols for built-in types on demand

Summary:
There is a reserved range of type indexes for built-in types (like integers).
This will create a symbol for a built-in type if the caller askes for one by
type index.  This is also plumbing for being able to recall symbols by type
index in general, but user-defined types will come in subsequent patches.

Reviewers: rnk, zturner

Subscribers: mgorny, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D35163

llvm-svn: 307834