cmd/compile: detect and diagnose invalid //go: directive placement

Thie CL changes cmd/compile/internal/syntax to give the gc half of
the compiler more control over pragma handling, so that it can prepare
better errors, diagnose misuse, and so on. Before, the API between
the two was hard-coded as a uint16. Now it is an interface{}.
This should set us up better for future directives.

In addition to the split, this CL emits a "misplaced compiler directive"
error for any directive that is in a place where it has no effect.
I've certainly been confused in the past by adding comments
that were doing nothing and not realizing it. This should help
avoid that kind of confusion.

The rule, now applied consistently, is that a //go: directive
must appear on a line by itself immediately before the declaration
specifier it means to apply to. See cmd/compile/doc.go for
precise text and test/directive.go for examples.

This may cause some code to stop compiling, but that code
was broken. For example, this code formerly applied the
//go:noinline to f (not c) but now will fail to compile:

	//go:noinline
	const c = 1

	func f() {}

Change-Id: Ieba9b8d90a27cfab25de79d2790a895cefe5296f
Reviewed-on: https://go-review.googlesource.com/c/go/+/228578
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/syntax.go

@@ -8,7 +8,6 @@ package gc
 
 import (
 	"cmd/compile/internal/ssa"
-	"cmd/compile/internal/syntax"
 	"cmd/compile/internal/types"
 	"cmd/internal/obj"
 	"cmd/internal/objabi"
@@ -483,7 +482,7 @@ type Param struct {
 	// OTYPE
 	//
 	// TODO: Should Func pragmas also be stored on the Name?
-	Pragma syntax.Pragma
+	Pragma PragmaFlag
 	Alias  bool // node is alias for Ntype (only used when type-checking ODCLTYPE)
 }
 
@@ -565,7 +564,7 @@ type Func struct {
 	Endlineno src.XPos
 	WBPos     src.XPos // position of first write barrier; see SetWBPos
 
-	Pragma syntax.Pragma // go:xxx function annotations
+	Pragma PragmaFlag // go:xxx function annotations
 
 	flags      bitset16
 	numDefers  int // number of defer calls in the function