[C++20][Modules] Handle defaulted and deleted functions in header units.

Address part of https://github.com/llvm/llvm-project/issues/60079.

Deleted and Defaulted functions are implicitly inline, but that state
is not set at the point that we perform the diagnostic checks for externally-
visible non-inline functions; check the function body type explicitly in the
diagnostic.

Differential Revision: https://reviews.llvm.org/D141908