libstdc++: Move COW string definitions to separate header

libstdc++: Move COW string definitions to separate header

This moves the definitions of the COW string to a separate file, so that
they don't need to be preprocessed for the common case. We could also
move the SSO string definitions to a new file, so that they don't need
to be preprocessed for the old ABI case, but that would require more
shovel work because there are some parts of <bits/basic_string.h> and
<bits/basic_string.tcc> that are common to both definitions.

libstdc++-v3/ChangeLog:

	* include/Makefile.am: Add new header.
	* include/Makefile.in: Regenerate.
	* include/bits/basic_string.h [!_GLIBCXX_USE_CXX11_ABI]
	(basic_string): Move definition of Copy-on-Write string to
	new file.
	* include/bits/basic_string.tcc: Likewise.
	* include/bits/cow_string.h: New file.