[NativePDB] Make GetOrCreateDeclForUid return an lldb CompilerDecl

We intend to make PdbAstBuilder abstract and implement
PdbAstBuilderClang along with any other languages that wish to use
PDBs. Thus, change GetOrCreateDeclForUid from returning a clang decl
to a lldb_private::CompilerDecl.

llvm-svn: 366650