Don't record string concatenation data for 'RESERVED_LOCATION_P'

Don't record string concatenation data for 'RESERVED_LOCATION_P'

'RESERVED_LOCATION_P' means 'UNKNOWN_LOCATION' or 'BUILTINS_LOCATION'.
We're using 'UNKNOWN_LOCATION' as a spare value for 'Empty', so should
ascertain that we don't use it as a key additionally.  Similarly for
'BUILTINS_LOCATION' that we'd later like to use as a spare value for
'Deleted'.

As discussed in the source code comment added, for these we didn't have
stable behavior anyway.

Follow-up to r239175 (commit 88fa5555a309e5d6c6171b957daaf2f800920869)
"On-demand locations within string-literals".

	gcc/
	* input.c (string_concat_db::record_string_concatenation)
	(string_concat_db::get_string_concatenation): Skip for
	'RESERVED_LOCATION_P'.
	gcc/testsuite/
	* gcc.dg/plugin/diagnostic-test-string-literals-1.c: Adjust
	expected error diagnostics.