diagnostics: add automatic URL-ification within messages

In r10-3781-gd26082357676a3 GCC's pretty-print framework gained
the ability to emit embedding URLs via escape sequences
for marking up text output..

In r10-3783-gb4c7ca2ef3915a GCC started using this for the
[-Wname-of-option] emitted at the end of each diagnostic so that it
becomes a hyperlink to the documentation for that option on the GCC
website.

This makes it much more convenient for the user to locate pertinent
documentation when a diagnostic is emitted.

The above involved special-casing in one specific place, but there is
plenty of quoted text throughout GCC's diagnostic messages that could
usefully have a documentation URL: references to options, pragmas, etc

This patch adds a new optional "urlifier" parameter to pp_format.
The idea is that a urlifier object has responsibility for mapping from
quoted strings in diagnostic messages to URLs, and pp_format has the
ability to automatically add URL escapes for strings that the urlifier
gives it URLs for.

For example, given the format string:

  "%<#pragma pack%> has no effect with %<-fpack-struct%>"

with this patch GCC is able to automatically linkify the "#pragma pack"
text to
  https://gcc.gnu.org/onlinedocs/gcc/Structure-Layout-Pragmas.html
and the "-fpack-struct" text to:
  https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#index-fpack-struct

and we don't have to modify the format string itself.

This is only done for the pp_format within
diagnostic_context::report_diagnostic i.e. just for the primary message
in each diagnostics, and not for other places within GCC that use pp
format internally.

"urlifier" is an abstract base class, with a GCC-specific subclass
implementing the logic for generating URLs into GCC's HTML
documentation via binary search in a data table.  This patch implements
the gcc_urlifier with a small table generated by hand; the data table in
this patch only covers various pragmas and the option referenced by the
above pragma message.

I have a followup patch that scripts the creation of this data by
directly scraping the output of "make html", thus automating all this,
and (I hope) minimizing the work of ensuring that documentation URLs
emitted by GCC match the generated documentation.

gcc/ChangeLog:
	* Makefile.in (GCC_OBJS): Add gcc-urlifier.o.
	(OBJS): Likewise.

gcc/c-family/ChangeLog:
	* c-pragma.cc:: (handle_pragma_push_options): Fix missing "GCC" in
	name of pragma in "junk" message.
	(handle_pragma_pop_options): Likewise.

gcc/ChangeLog:
	* diagnostic.cc: Include "pretty-print-urlifier.h".
	(diagnostic_context::initialize): Initialize m_urlifier.
	(diagnostic_context::finish): Clean up m_urlifier
	(diagnostic_report::diagnostic): m_urlifier to pp_format.
	* diagnostic.h (diagnostic_context::m_urlifier): New field.
	* gcc-urlifier.cc: New file.
	* gcc-urlifier.def: New file.
	* gcc-urlifier.h: New file.
	* gcc.cc: Include "gcc-urlifier.h".
	(driver::global_initializations): Initialize global_dc->m_urlifier.
	* pretty-print-urlifier.h: New file.
	* pretty-print.cc: Include "pretty-print-urlifier.h".
	(obstack_append_string): New.
	(urlify_quoted_string): New.
	(pp_format): Add "urlifier" param and use it to implement optional
	urlification of quoted text strings.
	(pp_output_formatted_text): Make buffer a const pointer.
	(selftest::pp_printf_with_urlifier): New.
	(selftest::test_urlification): New.
	(selftest::pretty_print_cc_tests): Call it.
	* pretty-print.h (class urlifier): New forward declaration.
	(pp_format): Add optional urlifier param.
	* selftest-run-tests.cc (selftest::run_tests): Call
	selftest::gcc_urlifier_cc_tests .
	* selftest.h (selftest::gcc_urlifier_cc_tests): New decl.
	* toplev.cc: Include "gcc-urlifier.h".
	(general_init): Initialize global_dc->m_urlifier.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>

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