go/internal/gcimporter: rename instType to instanceType

This was suggested in CL 349949, and is done in the standard library in
CL 350031.

Change-Id: I3e7e3a5f56026ec5a9df7acab3079dc92ea07efb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/350032
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

Author: findleyr

go/internal/gcimporter/iimport.go

@@ -74,7 +74,7 @@ const (
 	structType
 	interfaceType
 	typeParamType
-	instType
+	instanceType
 	unionType
 )
 
@@ -709,7 +709,7 @@ func (r *importReader) doType(base *types.Named) types.Type {
 		r.p.doDecl(pkg, name)
 		return r.p.tparamIndex[id]
 
-	case instType:
+	case instanceType:
 		if r.p.exportVersion < iexportVersionGenerics {
 			errorf("unexpected instantiation type")
 		}