[TLI] Unify LibFunc attribute inference. NFCI.

Now the pass is just a tiny wrapper around the util. This lets us reuse
the logic elsewhere (done here for BuildLibCalls) instead of duplicating
it.

The next step is to have something like getOrInsertLibFunc that also
sets the attributes.

Differential Revision: http://reviews.llvm.org/D19470

llvm-svn: 267759