[dev.inline] cmd/compile/internal/syntax: use syntax.Pos for all external positions

- use syntax.Pos in syntax.Error (rather than line, col)
- use syntax.Pos in syntax.PragmaHandler (rather than just line)
- update uses
- better documentation in various places

Also:
- make Pos methods use Pos receiver (rather than *Pos)

Reviewed in and cherry-picked from https://go-review.googlesource.com/#/c/33891/.
With minor adjustments to noder.go to make merge compile.

Change-Id: I5507cea6c2be46a7677087c1aeb69382d31033eb
Reviewed-on: https://go-review.googlesource.com/34236
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -77,9 +77,13 @@ func (s *source) error(msg string) {
 }
 
 // getr reads and returns the next rune.
-// If an error occurs, the error handler provided to init
-// is called with position (line and column) information
-// and error message before getr returns.
+//
+// If a read or source encoding error occurs, getr
+// calls the error handler installed with init.
+// The handler must exist.
+//
+// The (line, col) position passed to the error handler
+// is always at the current source reading position.
 func (s *source) getr() rune {
 redo:
 	s.r0, s.line0, s.col0 = s.r, s.line, s.col