go/gcexportdata: drop support for the ancient binary format

Compilers and tools haven't produced it since go1.11,
several years ago now.

Change-Id: I5056c5bba81030a2eba5e3931190b8249524aed7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/494442
gopls-CI: kokoro <[email protected]>
Run-TryBot: Alan Donovan <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

go/gcexportdata/gcexportdata.go

@@ -128,15 +128,14 @@ func Read(in io.Reader, fset *token.FileSet, imports map[string]*types.Package,
 	// (from "version"). Select appropriate importer.
 	if len(data) > 0 {
 		switch data[0] {
-		case 'i':
-			_, pkg, err := gcimporter.IImportData(fset, imports, data[1:], path)
-			return pkg, err
+		case 'v', 'c', 'd': // binary, till go1.10
+			return nil, fmt.Errorf("binary (%c) import format is no longer supported", data[0])
 
-		case 'v', 'c', 'd':
-			_, pkg, err := gcimporter.BImportData(fset, imports, data, path)
+		case 'i': // indexed, till go1.19
+			_, pkg, err := gcimporter.IImportData(fset, imports, data[1:], path)
 			return pkg, err
 
-		case 'u':
+		case 'u': // unified, from go1.20
 			_, pkg, err := gcimporter.UImportData(fset, imports, data[1:], path)
 			return pkg, err