diagnostic_metadata: unbreak xgettext (v2)

Changed in v2:
- rename from warning_with_metadata_at to warning_meta
- fix test plugins

While C++ can have overloads, xgettext can't deal with overloads that have
different argument positions, leading to two failures in "make gcc.pot":

emit_diagnostic_valist used incompatibly as both
--keyword=emit_diagnostic_valist:4
--flag=emit_diagnostic_valist:4:gcc-internal-format and
--keyword=emit_diagnostic_valist:5
--flag=emit_diagnostic_valist:5:gcc-internal-format

warning_at used incompatibly as both
--keyword=warning_at:3 --flag=warning_at:3:gcc-internal-format and
--keyword=warning_at:4 --flag=warning_at:4:gcc-internal-format

The emit_diagnostic_valist overload isn't used anywhere (I think it's
a leftover from an earlier iteration of the analyzer patch kit).

The warning_at overload is used throughout the analyzer but nowhere else.

Ideally I'd like to consolidate this argument with something
constructable in various ways:
- from a metadata and an int or
- from an int (or, better an "enum opt_code"),
so that the overload happens when implicitly choosing the ctor, but
that feels like stage 1 material.

In the meantime, fix xgettext by deleting the unused overload and
renaming the used one.

gcc/analyzer/ChangeLog:
	* region-model.cc (poisoned_value_diagnostic::emit): Update for
	renaming of warning_at overload to warning_meta.
	* sm-file.cc (file_leak::emit): Likewise.
	* sm-malloc.cc (double_free::emit): Likewise.
	(possible_null_deref::emit): Likewise.
	(possible_null_arg::emit): Likewise.
	(null_deref::emit): Likewise.
	(null_arg::emit): Likewise.
	(use_after_free::emit): Likewise.
	(malloc_leak::emit): Likewise.
	(free_of_non_heap::emit): Likewise.
	* sm-sensitive.cc (exposure_through_output_file::emit): Likewise.
	* sm-signal.cc (signal_unsafe_call::emit): Likewise.
	* sm-taint.cc (tainted_array_index::emit): Likewise.

gcc/ChangeLog:
	* diagnostic-core.h (warning_at): Rename overload to...
	(warning_meta): ...this.
	(emit_diagnostic_valist): Delete decl of overload taking
	diagnostic_metadata.
	* diagnostic.c (emit_diagnostic_valist): Likewise for defn.
	(warning_at): Rename overload taking diagnostic_metadata to...
	(warning_meta): ...this.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/diagnostic_plugin_test_metadata.c: Update for
	renaming of warning_at overload to warning_meta.
	* gcc.dg/plugin/diagnostic_plugin_test_paths.c: Likewise.