Combiners that use C++ code in their "apply" pattern only use that. They
never mix it with MIR patterns as that has little added value.
This patch restricts C++ apply code so that if C++ is used, we cannot
use MIR patterns or builtins with it. Adding this restriction allows us
to merge calls to match and apply C++ code together, which in turns
makes it so we can just have MatchData variables on the stack.
So before, we would have
```
GIM_CheckCxxInsnPredicate // match
GIM_CheckCxxInsnPredicate // apply
GIR_Done
```
Alongside a massive C++ struct holding the MatchData of all rules
possible (which was a big space/perf issue).
Now we just have
```
GIR_DoneWithCustomAction
```
And the function being ran just does
```
unsigned SomeMatchData;
if (match(SomeMatchData))
apply(SomeMatchData)
```
This approach solves multiple issues in one:
- MatchData handling is greatly simplified and more efficient, "don't
pay for what you don't use"
- We reduce the size of the match table
- Calling C++ code has a certain overhead (we need a switch), and this
overhead is only paid once now.
Handling of C++ code inside PatFrags is unchanged though, that still
emits a `GIM_CheckCxxInsnPredicate`. This is completely fine as they
can't use MatchDatas.
Refactor of the llvm-tblgen source into:
- a "Basic" library, which contains the bare minimum utilities to build
`llvm-min-tablegen`
- a "Common" library which contains all of the helpers for TableGen
backends. Such helpers can be shared by more than one backend, and even
unit tested (e.g. CodeExpander is, maybe we can add more over time)
Fixes#80647
