go/analysis/passes/printf: check anonymous functions too

This change causes the printf analyzer to consider anonymous
functions that are directly assigned to a variable, such as

   printf := func(format string, args ...any) { ... }

to be analyzed the same way as an ordinary function. This
allows mistakes calling local printf-wrapper functions to
be reported.

We denote printf-like functions by a types.Object (which
may be a *Func or, now, a *Var) throughout the algorithm.

This generalizes to local and global vars and struct fields,
even ones imported from another package, though since one
cannot export a fact for foreign symbols, the imported var
will only have the wrapper property within the unit that
encloses the assignment.

Also:
- a number of clarifications to identifier names, comments, etc:
- fix a couple of latent printf bugs in x/tools.

+ tests

Change-Id: I82c23dae4a682b574c2b0f4e6daa9be260ae47be
Reviewed-on: https://go-review.googlesource.com/c/tools/+/706635
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: adonovan

go/analysis/passes/nilness/nilness.go

@@ -52,7 +52,7 @@ func runFunc(pass *analysis.Pass, fn *ssa.Function) {
 	// notNil reports an error if v is provably nil.
 	notNil := func(stack []fact, instr ssa.Instruction, v ssa.Value, descr string) {
 		if nilnessOf(stack, v) == isnil {
-			reportf("nilderef", instr.Pos(), descr)
+			reportf("nilderef", instr.Pos(), "%s", descr)
 		}
 	}
 

go/analysis/passes/printf/doc.go

@@ -82,6 +82,16 @@
 //		...
 //	}
 //
+// A local function may also be inferred as a printf wrapper. If it
+// is assigned to a variable, each call made through that variable will
+// be checked just like a call to a function:
+//
+//	logf := func(format string, args ...any) {
+//		message := fmt.Sprintf(format, args...)
+//		log.Printf("%s: %s", prefix, message)
+//	}
+//	logf("%s", 123) // logf format %s has arg 123 of wrong type int
+//
 // # Specifying printf wrappers by flag
 //
 // The -funcs flag specifies a comma-separated list of names of