c++: Fix up and tweak std-name-hint.gperf

c++: Fix up and tweak std-name-hint.gperf

When converting reflection metafunction evaluation into gperf, I had
a look at the other *.gperf files in gcc/cp/.
cfns.gperf doesn't have any hash conflicts, but std-name-hint.gperf
has 7, which I believe means that for 7 of the strings it falls back to some
slow loop over not sure how many entries.

Apparently, 2 out of the 7 conflicts are just because two lines are
identically duplicated in the file, the patch included below fixes that.

2025-10-30  Jakub Jelinek  <jakub@redhat.com>

	* std-name-hint.gperf: Remove duplicate entries for ispanstream
	and istringstream.
	* std-name-hint.h: Regenerate.