The constraint check for filesystem::path construction uses
decltype(__is_path_src(declval<Source>())) which mean it considers
conversion from an rvalue.  When Source is a volatile-qualified type
it cannot use is_path_src(const Unknown&) because a const lvalue
reference can only bind to a non-volatile rvalue.

Since the relevant path members all have a const Source& parameter,
the constraint should be defined in terms of declval<const Source&>(),
not declval<Source>(). This avoids the problem of volatile-qualified
rvalues, because we no longer use an rvalue at all.

libstdc++-v3/ChangeLog:

	PR libstdc++/100630
	* include/bits/fs_path.h (__is_constructible_from): Test
	construction from a const lvalue, not an rvalue.
	* include/experimental/bits/fs_path.h (__is_constructible_from):
	Likewise.
	* testsuite/27_io/filesystem/path/construct/100630.cc: New test.
	* testsuite/experimental/filesystem/path/construct/100630.cc:
	New test.

Tested x86_64-linux, pushed to gcc-10 (this isn't needed for gcc-11 or
trunk, but I also plan to backport it to gcc-9).