The general strategy here is to force use of a thunk when we want to use
default arguments, and have Clang generate uses of the default arguments
on its side of the thunk.
To support this, change the key type used in `clang_decls` from being
just a `Decl*` to being a pair of `Decl*` and number of parameters in
the case of function decls. Import distinct `SemIR::Function`s for each
number of parameters that's used, and corresponding distinct thunks.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Mark C++ functions as used when overload resolution selects them, and
trigger Clang's end-of-TU processing at the end of the Carbon
compilation to perform instantiation and other pending cleanup steps.
Richard SmithandJon Ross-Perkins