analyzer: rename region-model-impl-calls.cc to kf.cc

gcc/ChangeLog:
	* Makefile.in (ANALYZER_OBJS): Update for renaming of
	analyzer/region-model-impl-calls.cc to analyzer/kf.cc.

gcc/analyzer/ChangeLog:
	* analyzer.h (class known_function): Expand comment.
	* region-model-impl-calls.cc: Rename to...
	* kf.cc: ...this.
	* known-function-manager.h (class known_function_manager): Add
	leading comment.

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