internal/lsp: use bytes.Buffer instead of strings.Builder

x/tools only supports Go 1.10 and Go 1.11, but it's such a simple fix to
support 1.9, so change from strings.Builder to bytes.Buffer.

Change-Id: Ie37d6c7da7ce7dbbd4e9ec933e1eff3304142a59
Reviewed-on: https://go-review.googlesource.com/c/149178
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

internal/lsp/source/completion.go

@@ -1,6 +1,7 @@
 package source
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"go/ast"
@@ -105,7 +106,6 @@ func completions(file *ast.File, pos token.Pos, fset *token.FileSet, pkg *types.
 	if items, ok := complit(path, pos, pkg, info, found); ok {
 		return items, "", nil
 	}
-
 	switch n := path[0].(type) {
 	case *ast.Ident:
 		// Set the filter prefix.
@@ -440,7 +440,7 @@ func formatType(typ types.Type, qualifier types.Qualifier) (detail string, kind
 
 // formatParams correctly format the parameters of a function.
 func formatParams(t *types.Tuple, variadic bool, qualifier types.Qualifier) string {
-	var b strings.Builder
+	var b bytes.Buffer
 	b.WriteByte('(')
 	for i := 0; i < t.Len(); i++ {
 		if i > 0 {