The expression ctx._M_indent is not a constant expression when ctx is a
reference parameter, even though _M_indent is an enumerator. Rename it
to _S_indent to be consistent with our conventions, and refer to it as
PrintContext::_S_indent to be valid C++ code (at least until P2280 is
accepted as a DR).

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/101937
	* src/c++11/debug.cc (PrintContext::_M_indent): Replace with a
	static data member.
	(print_word): Use qualified-id to access it.

Tested x86_64-linux. Committed to trunk.