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`).
