[CIR][NFC] Replace uses of isa/dyn_cast/cast/... member functions

[jopperm]

Mechanical rewrite to use the corresponding free functions; fixes #702.

I used a slightly modified version of the clang-tidy check provided in https://discourse.llvm.org/t/psa-deprecating-cast-isa-methods-in-some-classes/70909 to rewrite the C++ source files, regular expressions for the TableGen files, and manual cleanups where needed (e.g. chains like x.foo().cast<A>().bar().cast<B>())

I applied the following heuristic to determine which namespace prefix to use:

• If the target type is not qualified, and the TU has using namespace mlir or the code is inside the mlir namespace -> use a plain isa/cast/...
  • Exception: Always qualify inside custom types and attributes, because their base classes define the very members we want to get rid of.
• Else. i.e. the target type is qualified as ::mlir:: or mlir::, use that prefix.

The clang-tidy check also rewrote dyn_cast_or_null to dyn_cast_if_present. I think that's useful because the former variant is going to be deprecated as well in the future.

I'm using -Werror=deprecated-declarations to test the change (see 6b7420a93278ee01d37d95882dec39358378cfb3); this required also changing two occurrences of StringRef::equals to ==. I could also just drop the commit here; maybe we want to enable -Werror in general (there aren't too many other warnings left in the codebase).

[seven-mile]

Note that /we4996 may be too aggressive for MSVC.

See the discussion in this issue.

Currently HEAD won't compile with the toolchain of VS2022, which use newer MSSTL with this check introduced in June 2022.

[jopperm]

Thanks for investigating! So until the use of std::complex<APInt|APFloat> is removed upstream or Microsoft changes their STL implementation, we would have three options:

1. Revert enforcing the check for deprecated API in the build, document this in the coding guidelines, and catch it in code review.
2. Only do -Werror=deprecated-declarations on Linux (after all, should be enough to check the code on one platform in the CI).
3. Define _SILENCE_NONFLOATING_COMPLEX_DEPRECATION_WARNING when compiling on Windows to work around the issue.