Attach attribute "disable-tail-calls" to the functions in the IR.

This commit adds back the code that seems to have been dropped unintentionally
in r176985.

rdar://problem/13752163

Differential Revision: http://reviews.llvm.org/D10100


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239426 91177308-0d34-0410-b5e6-96231b3b80d8