Make EmitFunctionHeader virtual

Make EmitFunctionHeader virtual

This is to fix problems introduced by r232481. For HSAIL,
this function does essentially nothing desirable, and
injects unwanted / incorrect stuff before the function.
The only thing it really needs to do is call EmitFunctionEntryLabel
in this case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238222 91177308-0d34-0410-b5e6-96231b3b80d8