Add PIC-level support to Clang.

Summary:
This distinguishes between -fpic and -fPIC now, with the additions in LLVM for
PIC level support.

Test Plan: No regressions

Reviewers: echristo, rafael

Reviewed By: rafael

Subscribers: rnk, emaste, llvm-commits

Differential Revision: http://reviews.llvm.org/D5400