Recommit "Support attribute used in member funcs of class templates"

This diff previously exposed a bug in LLVM's IRLinker, breaking
buildbots that tried to self-host LLVM with monolithic LTO.
The bug is now in LLVM by D59552

Original commit message:
As PR17480 describes, clang does not support the used attribute
for member functions of class templates. This means that if the member
function is not used, its definition is never instantiated. This patch
changes clang to emit the definition if it has the used attribute.

Test Plan: Added a testcase

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D56928

llvm-svn: 356598