Make filesystem::path safe for self assignment

Make filesystem::path safe for self assignment

The standard says "If *this and p are the same object, has no effect."
Previously we ended up clearing the path.

	* include/bits/fs_path.h (path::operator=(path&&)): Check for self
	assignment.
	* src/c++17/fs_path.cc (path::operator=(const path&)): Likewise.
	* testsuite/27_io/filesystem/path/assign/copy.cc: Test self
	assignment.

From-SVN: r270171