Add SymbolVendor::GetMainFileSpec and simplify CommandObjectTargetModulesList::PrintModule

Summary:
Add SymbolVendor::GetMainFileSpec and simplify CommandObjectTargetModulesList::PrintModule.

All tests pass on OS X.

Reviewers: abidh, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits, clayborg, abidh

Differential Revision: http://reviews.llvm.org/D8002

llvm-svn: 231849