[NativePDB] Make GetTranslationUnitDecl return an lldb CompilerDeclCtx

Summary:
We intend to make PdbAstBuilder abstract and implement
PdbAstBuilderClang along with any other languages that wish to use
PDBs. This is the first step.

Differential Revision: https://reviews.llvm.org/D64852

llvm-svn: 366293