[MC] Fold A-B when A's fragment precedes B's fragment

When the MCAssembler is non-null and the MCAsmLayout is null, we can fold A-B
when

* A and B are in the same fragment, or
* A's fragment suceeds B's fragment, and they are not separated by non-data fragments (D69411)

This patch allows folding when A's fragment precedes B's fragment so
that `9997b - . == 0` below can be evaluated as true:

```
nop
.arch_extension sec
9997:nop
// old behavior: error: expected absolute expression
.if 9997b - . == 0
.endif
```

Add a case to llvm/test/MC/ARM/directive-if-subtraction.s.
Note: for MCAsmStreamer, we cannot evaluate `.if . - 9997b == 0` at parse
time due to MCAsmStreamer::getAssemblerPtr returning nullptr (D45164).

Some Darwin tests check that this folding does not work. Add `.p2align 2` to
block some label difference folding or adjust the tests.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D153096