[CXX Interoperability] Stop re-importing FuncDecl when FuncDecl is imported before parent record

[Huddie]

We saw a test case failing when 2 records contain the same operator. This occurs because when the first operator is called, we import the record associated with that operator but we also import the function for the 2nd record. So if we have 2 records Foo and Bar and both implement operator-, after calling Foo's operator- we would have imported

1. Foo
2. Foo.operator-
3. Bar.operator-

Then when we call Bar.operator- we try importing Bar record & then import the operator again. So that ends up with

1. Foo
2. Foo.operator-
3. Bar.operator-
4. Bar
5. Bar.operator-

which causes there to be 2 imports of the same operator (FuncDecl)

This patch checks to see if the FuncDecl was previously imported and returns early if it has been

Thanks @egorzhdan and @zoecarver for helping me debug this one :p

[Huddie]

@swift-ci please smoke test

[zoecarver]

Really glad you figured it out! Thanks, Ehud!

[egorzhdan]

Looks great, thanks!
@Huddie can you please remove the merge commit from this branch? It will mess up git blame if we merge this. You can rebase your branch onto main instead.

[Huddie]

@swift-ci please smoke test

[Huddie]

@swift-ci please smoke test

[Huddie]

@swift-ci please test windows

[Huddie]

@swift-ci please smoke test

[egorzhdan]

Thanks!