analyzer: getchar has no side-effects

analyzer: getchar has no side-effects

Seen whilst debugging another issue, where the analyzer was assuming
conservatively that a call to getchar could clobber a global.

This is handled for most of the other stdio functions by the list
in sm-file.cc

gcc/analyzer/ChangeLog:
	* region-model.cc (region_model::on_call_pre): Treat getchar as
	having no side-effects.

gcc/testsuite/ChangeLog:
	* gcc.dg/analyzer/getchar-1.c: New test.