Sema: Diagnose global replacement functions declared as inline

This fixes PR17591.

N.B. This actually goes beyond what the standard mandates by requiring
the restriction to hold for declarations instead of definitions.  This
is believed to be a defect in the standard and an LWG issue has been
submitted.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193044 91177308-0d34-0410-b5e6-96231b3b80d8