[ADT][NFC] Fix compilation of headers under C++23

`DoubleAPFloat` has a `unique_ptr<APFloat[]>` member. In
`DoubleAPFloat::operator=` and `DoubleAPFloat::get{First,Second}`,
the methods of this unique_ptr are getting instantiated. At that
point `APFloat` is still only a forward declaration.

This triggers undefined behavior. So far, we were probaly just
lucky and the code compiled fine. However, with C++23
`std::unique_ptr` became constexpr, and clang (and other compilers) are
now diagnosing this latent bug as an error.

This commit fixes the issue by moving the function definitions
out of the class definition of `DoubleAPFloat`, after the declaration
of `APFloat`.

A similar issue exists in `ModuleSummaryIndex.h`, the fix is pretty
much identical.

Fixes #59784

Differential Revision: https://reviews.llvm.org/D149854

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