Mark calls in thunk functions as tail-call optimization candidates

When a thunk is generated with a call to the original adjusted function, 
the thunk appears in the debugger call stack. We want the backend to perform
tail-call optimization on the call, to make it invisible to the debugger.

This fixes PR24235

Patch by: amjad.aboud@intel.com
Differential Revision: http://reviews.llvm.org/D11476

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@244207 91177308-0d34-0410-b5e6-96231b3b80d8