[MetaRenamer] Don't rename library functions.

Library functions can have specific semantics that affect the behavior of
certain passes. DSE, for instance, gives special treatment to malloc-ed pointers
but not to pointers returned from an equivalently typed (but differently named)
function.

MetaRenamer ought not to alter program semantics, so library functions must
remain untouched.

Reviewers: mehdi_amini, majnemer, chandlerc, davide

Reviewed By: davide

Subscribers: davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D31304

llvm-svn: 298659