compiletest: don't register predefined `MSVC`/`NONMSVC` FileCheck prefixes
This was fragile as it was based on host target passed to compiletest,
but the user could cross-compile and run test for a different target
(e.g. cross from linux to msvc, but msvc won't be set on the target).
Furthermore, it was also very surprising as normally revision names
(other than `CHECK`) was accepted as FileCheck prefixes.
This partially reverts the `MSVC`/`NONMSVC` predefined FileCheck
prefix registration introduced historically for some codegen tests.
This makes some codegen tests more verbose since they now need to
explicitly introduce `MSVC`/`NONMSVC` revisions, but I think that's
less surprising, e.g.:
```rs
//@ revisions: MSVC NONMSVC
//`@[MSVC]` only-msvc
//`@[NONMSVC]` ignore-msvc
```
Note that revisions are not *only* FileCheck prefixes in
FileCheck-based test suites, as they also can be used
to conditionally apply certain compiletest directives.
r? `@Zalathar` (or reroll a `r/? compiletest` reviewer)
try-job: x86_64-msvc
try-job: i686-msvc
try-job: x86_64-mingw-1
try-job: i686-mingw
b9784988f7