Some autoconf tests use module level inline asm to test compiler's handling of

Some autoconf tests use module level inline asm to test compiler's handling of
.cfi_startproc. e.g. libffi:

 $ cat confopt.c
asm (".cfi_startproc\n\t.cfi_endproc");

int main () { return 0; }

Teach MC / dwarf emission to handle these cfi directives which essentially
create an empty frame.

rdar://10017184

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@138504 91177308-0d34-0410-b5e6-96231b3b80d8