Kill unused variable m_tu_decl_up in SymbolFilePDB

Summary:
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
specific code.

Reviewers: rnk, zturner, compnerd

Reviewed By: compnerd

Subscribers: jdoerfert

Differential Revision: https://reviews.llvm.org/D59804

llvm-svn: 357113