Temporarily disable the checks for mutable fields in a C++ struct. A better solution would be to use some heuristic to detect if a method mutates a mutable field, and allow the user to adjust mutability with the existing `nonmutating` annotation.
rdar://92621793
Previously, the clang importer marked all const methods as mutating whenever a C++ record had mutable fields. This change allows overriding this behavior by using the "nonmutating" swift_attr attribute.
Fixes SR-15907.
This change teaches ClangImporter to import C++ methods marked with `__attribute__((__swift_attr__("mutating")))` as mutating in Swift. This is useful, for example, when a method mutates `this` despite being `const` in C++ (e.g. via `const_cast`).
