[clang] [clang][ASTMatcher] Add matchers for CXXFoldExpr (PR #71245)

[Julian Schmidt via cfe-commits]

5chmidti wrote:

> Pull request doesn't say anything.

Fixed, I just listed the matchers that were added/adjusted for `CXXFoldExpr` support.

> Whats a purpose of those matchers (except only to have them).

Sorry, forgot to add that comment. I didn't want that to be part of the pr description (because of the squashed commit message), or should it?
My fix for #70323 uses `cxxFoldExpr` and `hasFoldInit`. `ASTImporterTest.cpp` already defined matchers for fold expressions, which is why I thought that they could be part of the public matchers. If that is not enough to warrant adding these matchers to the publically available ones, I can just implement those two matchers in `ExprMutationAnalyzer.cpp`.

> Someone said not so long ago that we do not want to add matchers unless there is some usecase for them in some of the tools.

I think I read the same comment, but I don't know who said that and where.

https://github.com/llvm/llvm-project/pull/71245