Don't emit prologues or epilogues for naked functions (PR18791, PR20028)

For naked functions with parameters, Clang would still emit stores in the prologue
that would clobber the stack, because LLVM doesn't set up a stack frame. (This
shows up in -O0 compiles, because the stores are optimized away otherwise.)

For example:

  __attribute__((naked)) int f(int x) {
    asm("movl $42, %eax");
    asm("retl");
  }

Would result in:

  _Z1fi:
  movl    12(%esp), %eax
  movl    %eax, (%esp)    <--- Oops.
  movl    $42, %eax
  retl

Differential Revision: http://reviews.llvm.org/D5183

llvm-svn: 217198