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

llvm-svn: 138504