[SymbolFilePDB] Minor cleanup

Summary:
 - Remove unused code

- Adding `break` statement conditionally

- Ignore empty strings in FindTypeByName

Reviewers: zturner, rnk, lldb-commits

Reviewed By: zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44165

llvm-svn: 326870