[FileCheck] Support comment directives

Sometimes you want to disable a FileCheck directive without removing
it entirely, or you want to write comments that mention a directive by
name.  The `COM:` directive makes it easy to do this.  For example,
you might have:

```
; X32: pinsrd_1:
; X32:    pinsrd $1, 4(%esp), %xmm0

; COM: FIXME: X64 isn't working correctly yet for this part of codegen, but
; COM: X64 will have something similar to X32:
; COM:
; COM:   X64: pinsrd_1:
; COM:   X64:    pinsrd $1, %edi, %xmm0
```

Without this patch, you need to use some combination of rewording and
directive syntax mangling to prevent FileCheck from recognizing the
commented occurrences of `X32:` and `X64:` above as directives.
Moreover, FileCheck diagnostics have been proposed that might complain
about the occurrences of `X64` that don't have the trailing `:`
because they look like directive typos:

  <http://lists.llvm.org/pipermail/llvm-dev/2020-April/140610.html>

I think dodging all these problems can prove tedious for test authors,
and directive syntax mangling already makes the purpose of existing
test code unclear.  `COM:` can avoid all these problems.

This patch also updates the small set of existing tests that define
`COM` as a check prefix:

- clang/test/CodeGen/default-address-space.c
- clang/test/CodeGenOpenCL/addr-space-struct-arg.cl
- clang/test/Driver/hip-device-libs.hip
- llvm/test/Assembler/drop-debug-info-nonzero-alloca.ll

I think lit should support `COM:` as well.  Perhaps `clang -verify`
should too.

Reviewed By: jhenderson, thopre

Differential Revision: https://reviews.llvm.org/D79276

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