cmd/compile/internal/syntax: simpler position base update for line directives (cleanup)

The existing code was somewhat convoluted and made several assumptions
about the encoding of position bases:

1) The position's base for a file contained a position whose base
   pointed to itself (which is true but an implementation detail
   of src.Pos).

2) Updating the position base for a line directive required finding
   the base of the most recent's base position.

This change simply stores the file's position base and keeps using it
directly for each line directive (instead of getting it from the most
recently updated base).

Change-Id: I4d80da513bededb636eab0ce53257fda73f0dbc0
Reviewed-on: https://go-review.googlesource.com/95736Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -16,12 +16,13 @@ const debug = false
 const trace = false
 
 type parser struct {
-	base  *src.PosBase
+	file  *src.PosBase
 	errh  ErrorHandler
 	fileh FilenameHandler
 	mode  Mode
 	scanner
 
+	base   *src.PosBase // current position base
 	first  error        // first error encountered
 	errcnt int          // number of errors encountered
 	pragma Pragma       // pragma flags
@@ -31,8 +32,8 @@ type parser struct {
 	indent []byte // tracing support
 }
 
-func (p *parser) init(base *src.PosBase, r io.Reader, errh ErrorHandler, pragh PragmaHandler, fileh FilenameHandler, mode Mode) {
-	p.base = base
+func (p *parser) init(file *src.PosBase, r io.Reader, errh ErrorHandler, pragh PragmaHandler, fileh FilenameHandler, mode Mode) {
+	p.file = file
 	p.errh = errh
 	p.fileh = fileh
 	p.mode = mode
@@ -65,6 +66,7 @@ func (p *parser) init(base *src.PosBase, r io.Reader, errh ErrorHandler, pragh P
 		directives,
 	)
 
+	p.base = file
 	p.first = nil
 	p.errcnt = 0
 	p.pragma = 0
@@ -113,7 +115,7 @@ func (p *parser) updateBase(line, col uint, text string) {
 	}
 
 	// TODO(gri) pass column n2 to NewLinePragmaBase
-	p.base = src.NewLinePragmaBase(src.MakePos(p.base.Pos().Base(), line, col), filename, absFilename, uint(n) /*uint(n2)*/)
+	p.base = src.NewLinePragmaBase(src.MakePos(p.file, line, col), filename, absFilename, uint(n) /*uint(n2)*/)
 }
 
 func commentText(s string) string {