Delegate path operations to FileSpec.

Summary:
- Added PrependPathComponent utility functions to FileSpec.
- Delegate path operations in ParseCompileUnit to FileSpec.
- Delegate path operations in ParseSupportFiles to FileSpec.

Reviewers: clayborg, vharron, ovyalov

Reviewed By: ovyalov

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D10253

llvm-svn: 239127