Methods that are marked as `const` in C++ can still modify a field of the object if the field is marked as `mutable`.
We previously imported all methods of structs with mutable fields as mutating in Swift. Unfortunately this doesn't work well with libstdc++, which uses mutable fields for some commonly used container types.
Our current user model assumes that we trust the `const` keyword on a C++ method, and import them as non-mutating in Swift. Let's make sure the tests reflect that.
If possible, add imported members to the StructDecl's LookupTable rather than adding them directly as members. This will fix the issues with ordering that #39436 poorly attempted to solve during IRGen.
This also allows us to break out most of the test changes from #39436.
This change makes ClangImporter import some C++ member functions as non-mutating, given that they satisfy two requirements:
* the function itself is marked as `const`
* the parent struct doesn't contain any `mutable` members
`get` accessors of subscript operators are now also imported as non-mutating if the C++ `operator[]` satisfies the requirements above.
Fixes SR-12795.
