tree-optimization/106912 - clear const attribute from fntype

The following makes sure that after clearing pure/const from
instrumented function declarations we are adjusting call statements
fntype as well to handle indirect calls and because gimple_call_flags
looks at both decl and fntype.

Like the pure/const flag clearing on decls we refrain from touching
calls to known functions that do not have a body in the current TU.

	PR tree-optimization/106912
	* tree-profile.cc (tree_profiling): Update stmts only when
	profiling or testing coverage.  Make sure to update calls
	fntype, stripping 'const' there.

	* gcc.dg/profile-generate-4.c: New testcase.