This reflects the fact that the attribute's only for compiler-internal use, and isn't really equivalent to C's asm attribute, since it doesn't change the calling convention to be C-compatible.
...and then honor them.
While here, make -l a little more flexible (see interpret_with_options test).
rdar://problem/17830826, which unblocks the LLDB feature for the same.
Swift SVN r24033
