Rewrite the unsafe_declval function to work well with clang in C++26 …

[apolukhin]

…mode and that is able to return prvalues

Looks like static_cast<T>(*ptr) to prvalue now fails on clang in C++26. Changed it to function pointer call

Fixes: https://github.com/boostorg/pfr/issues/208 Probably fixes: https://github.com/boostorg/pfr/issues/173, https://github.com/boostorg/pfr/issues/110, https://github.com/boostorg/pfr/issues/126

[coveralls]

Pull Request Test Coverage Report for Build 15783550915

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 15761583817:	0.0%
Covered Lines:	405
Relevant Lines:	405

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💛 - Coveralls