pch: Fix streaming of strings with embedded null bytes

When a GTY'ed struct is streamed to PCH, any plain char* pointers it contains
(whether they live in GC-controlled memory or not) will be marked for PCH
output by the routine gt_pch_note_object in ggc-common.cc. This routine
special-cases plain char* strings, and in particular it uses strlen() to get
their length. Thus it does not handle strings with embedded null bytes, but it
is possible for something PCH cares about (such as a string literal token in a
macro definition) to contain such embedded nulls. To fix that up, add a new
GTY option "string_length" so that gt_pch_note_object can be informed the
actual length it ought to use, and use it in the relevant libcpp structs
(cpp_string and ht_identifier) accordingly.

gcc/ChangeLog:

	* gengtype.cc (output_escaped_param): Add missing const.
	(get_string_option): Add missing check for option type.
	(walk_type): Support new "string_length" GTY option.
	(write_types_process_field): Likewise.
	* ggc-common.cc (gt_pch_note_object): Add optional length argument.
	* ggc.h (gt_pch_note_object): Adjust prototype for new argument.
	(gt_pch_n_S2): Declare...
	* stringpool.cc (gt_pch_n_S2): ...new function.
	* doc/gty.texi: Document new GTY((string_length)) option.

libcpp/ChangeLog:

	* include/cpplib.h (struct cpp_string): Use new "string_length" GTY.
	* include/symtab.h (struct ht_identifier): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/pch/pch-string-nulls.C: New test.
	* g++.dg/pch/pch-string-nulls.Hs: New test.