Use the `Base` type alias from https://github.com/llvm/llvm-project/pull/158433.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

This commit marks the type converter in `populate...` functions as
`const`. This is useful for debugging.

Patterns already take a `const` type converter. However, some
`populate...` functions do not only add new patterns, but also add
additional type conversion rules. That makes it difficult to find the
place where a type conversion was added in the code base. With this
change, all `populate...` functions that only populate pattern now have
a `const` type converter. Programmers can then conclude from the
function signature that these functions do not register any new type
conversion rules.

Also some minor cleanups around the 1:N dialect conversion
infrastructure, which did not always pass the type converter as a
`const` object internally.

This patch is part of a project to move the Presburger library into
LLVM.

MLIR can't really be const-correct (it would need a `ConstValue` class
alongside the `Value` class really, like `ArrayRef` and
`MutableArrayRef`). This is however making is more consistent: method
that are directly modifying the Value shouldn't be marked const.

When converting branches, legalize target block arguments first.

Fixes llvm#70813

Do not match on illegal destination blocks. Also apply some minor
cleanups.

TODO: We should add region/block argument type conversions to properly
address this.

Issue: https://github.com/llvm/llvm-project/issues/70813

Tested with `check-mlir` and `check-mlir-integration`.

Issue: https://github.com/llvm/llvm-project/issues/56863

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D134620

This dialect is intended to model lower level/branch based control-flow constructs. The initial set
of operations are: AssertOp, BranchOp, CondBranchOp, SwitchOp; all split out from the current
standard dialect.

See https://discourse.llvm.org/t/standard-dialect-the-final-chapter/6061

Differential Revision: https://reviews.llvm.org/D118966