internal/goversion: update to 1.15

This is a second attempt at CL 230024, with
cmd/go/testdata/script/mod_retention.txt updated to perform a
version-independent comparison on the 'go' version added to a go.mod
file that lacks one.

Fixes #38708

Change-Id: I15dcd83b51ed5ec57946b419bcbaec41e85a46f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/230382
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: Bryan C. Mills

src/cmd/go/testdata/script/mod_retention.txt

@@ -64,7 +64,7 @@ cmp go.mod go.mod.tidy
 # However, that should not remove other redundant requirements.
 cp go.mod.nogo go.mod
 go list all
-cmp go.mod go.mod.redundant
+cmpenv go.mod go.mod.currentgo
 
 
 -- go.mod.tidy --
@@ -133,3 +133,13 @@ require (
 	rsc.io/sampler v1.3.0 // indirect
 	rsc.io/testonly v1.0.0 // indirect
 )
+-- go.mod.currentgo --
+module m
+
+go $goversion
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/sampler v1.3.0 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)

src/go/build/doc.go

@@ -112,6 +112,7 @@
 //	- "go1.12", from Go version 1.12 onward
 //	- "go1.13", from Go version 1.13 onward
 //	- "go1.14", from Go version 1.14 onward
+//	- "go1.15", from Go version 1.15 onward
 //	- any additional words listed in ctxt.BuildTags
 //
 // There are no build tags for beta or minor releases.

src/internal/goversion/goversion.go

@@ -10,4 +10,4 @@ package goversion
 //
 // When incrementing this, also add to the list at src/go/build/doc.go
 // (search for "onward").
-const Version = 14
+const Version = 15