Introduce symbol cache to PDB NativeSession

Instead of creating symbols directly in the findChildren methods of the native
symbol implementations, they will rely on the NativeSession to act as a factory
for these types.  This lets NativeSession cache the NativeRawSymbols in its
new symbol cache and makes that cache the source of unique IDs for the symbols.

Right now, this affects only NativeCompilandSymbols.  There's no external
change yet, so I think the existing tests are still sufficient.  Coming soon
are patches to extend this to built-in types and enums.

llvm-svn: 306610