llvm_config: use TARGET_CFX_CLANG_VERSION for includes

Summary: Forgot to change this when we introduced TARGET_CFX_CLANG_VERSION. Most triplets have this set to 3.5, however we were still using the 3.4 headers

Test Plan: verified

Reviewers: randall.rushing, jriordan001, anders3408, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D261