Keep empty assembly macro argument values in the middle of the list.

Empty macro arguments at the end of the list should be as-if not specified at
all, but those in the middle of the list need to be kept so as not to screw
up the positional numbering. E.g.:
.macro foo
foo_-bash___:
  nop
.endm

foo 1, 2, 3, 4
foo 1, , 3, 4

Should create two labels, "foo_1_2_3_4" and "foo_1__3_4".

rdar://11948769

llvm-svn: 161002