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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161002 91177308-0d34-0410-b5e6-96231b3b80d8