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.
* Update tests that relied on old behavior.
* Use mangleCXXName instead of mangleCXXCtor.
* Call VisitCXXRecordDecl not VisitRecordDecl from
VisitClassTemplateSpecializationDecl. This allows template constructors
to be imported and called correctly.
