[PartialInliner] Skip call sites where inlining fails.

Summary:
InlineFunction can fail, for example when trying to inline vararg
fuctions. In those cases, we do not want to bump partial inlining
counters or set AnyInlined to true, because this could leave an unused
function hanging around.

Reviewers: davidxl, davide, gyiu

Reviewed By: davide

Subscribers: llvm-commits, eraman

Differential Revision: https://reviews.llvm.org/D39581

llvm-svn: 317314