libstdc++: Fix filesystem::path construction from COW string [PR 99805]

libstdc++: Fix filesystem::path construction from COW string [PR 99805]

Calling the non-const data() member on a COW string makes it "leaked",
possibly resulting in reallocating the string to ensure a unique owner.

The path::_M_split_cmpts() member parses its _M_pathname string using
string_view objects and then calls _M_pathname.data() to find the offset
of each string_view from the start of the string. However because
_M_pathname is non-const that will cause a COW string to reallocate if
it happens to be shared with another string object. This results in the
offsets calculated for each component being wrong (i.e. undefined)
because the string views no longer refer to substrings of the
_M_pathname member. The fix is to use the parse.offset(c) member which
gets the offset safely.

The bug only happens for the path(string_type&&) constructor and only
for COW strings. When constructed from an lvalue string the string's
contents are copied rather than just incrementing the refcount, so
there's no reallocation when calling the non-const data() member. The
testsuite changes check the lvalue case anyway, because we should
probably change the deep copying to just be a refcount increment (by
adding a path(const string_type&) constructor or an overload for
__effective_range(const string_type&), for COW strings only).

libstdc++-v3/ChangeLog:

	PR libstdc++/99805
	* src/c++17/fs_path.cc (path::_M_split_cmpts): Do not call
	non-const member on _M_pathname, to avoid copy-on-write.
	* testsuite/27_io/filesystem/path/decompose/parent_path.cc:
	Check construction from strings that might be shared.