[SymbolFilePDB] Add support for function symbols

Summary:
This is combination of following changes,

- Resolve function symbols in PDB symbol file. `lldb-test symbols` will display information about function symbols.

- Implement SymbolFilePDB::FindFunctions methods. On lldb console, searching function symbol by name and by regular expression are both available.

- Create lldb type for PDBSymbolFunc.

- Add tests to check whether functions with the same name but from different sources can be resolved correctly.

Reviewers: zturner, lldb-commits

Reviewed By: zturner

Subscribers: amccarth, labath, llvm-commits

Differential Revision: https://reviews.llvm.org/D42443

llvm-svn: 324707