libstdc++: Force-inline shared_ptr::operator bool() for C++20 [PR108636]

libstdc++: Force-inline shared_ptr::operator bool() for C++20 [PR108636]

This avoids a linker error with -fkeep-inline-functions when including
<filesystem>. We can't backport the fix from trunk because it adds an
export to the shared library. By marking the "missing" symbol
always_inline for C++20 mode we don't need a definition in the library.

libstdc++-v3/ChangeLog:

	PR libstdc++/108636
	* include/bits/shared_ptr_base.h (__shared_ptr::operator bool):
	Add always_inline attribute for C++20 and later.