Kill unused variable m_tu_decl_up in SymbolFilePDB
An TranslationUnitDecl was being brought in from the clang::ASTContext
which required clang specific code to exist in SymbolFilePDB.
Since it was unused we can just get rid of it along with the clang
Reviewers: rnk, zturner, compnerd
Reviewed By: compnerd
Differential Revision: https://reviews.llvm.org/D59804