Add support for function attribute 'disable_tail_calls'.

The ``disable_tail_calls`` attribute instructs the backend to not
perform tail call optimization inside the marked function.

For example, 

int callee(int);

int foo(int a) __attribute__((disable_tail_calls)) {
  return callee(a); // This call is not tail-call optimized.
}

Note that this attribute is different from 'not_tail_called', which
prevents tail-call optimization to the marked function.

rdar://problem/8973573

Differential Revision: http://reviews.llvm.org/D12547


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@252986 91177308-0d34-0410-b5e6-96231b3b80d8