[Clang importer] Eliminate redundant imports of C++ fields as properties

[DougGregor]

A recent refactoring uncovered two places where we could end up importing a C++ field declaration as a property more than once:

1. Importing the declaration context of a field in C++ mode can then go import all of the fields. In such a case, check that the field we're importing didn't happen already, and bail out early if it did. This is common practice in the Clang importer but wasn't happening here.
2. One caller to the function that imported a field from a C++ base class into its inheriting class (as a computed property) wasn't checking the cache, and therefore created a redundant version.

Fix both issues.

[DougGregor]

@swift-ci please smoke test and merge