[inline Cost] Don't mark functions accessing varargs as non-inlinable

Summary:
rL323619 marks functions that are calling va_end as not viable for
inlining. This patch reverses that since this va_end doesn't need
access to the vriadic arguments list that are saved on the stack, only
va_start does.

Reviewers: efriedma, fhahn

Reviewed By: fhahn

Subscribers: eraman, haicheng, llvm-commits

Differential Revision: https://reviews.llvm.org/D52067

llvm-svn: 342675