internal/literals: re-enable the seed obfuscator

Now that we dropped Go 1.19, we can use it again,
since the referenced bug was fixed in Go 1.20.

This is a separate commit, as this change does alter the way we
obfuscate Go builds, so it's not just a cleanup.

internal/literals/obfuscators.go

@@ -22,7 +22,7 @@ var (
 		swap{},
 		split{},
 		shuffle{},
-		// seed{}, TODO: re-enable once https://go.dev/issue/47631 is fixed in Go 1.20
+		seed{},
 	}
 
 	TestObfuscator         string

testdata/script/literals.txtar

@@ -44,9 +44,8 @@ grep '^\s+\w+ \^= \w+ \* \w+$' debug1/test/main/extra_literals.go
 # Note that the line obfuscator adds an inline comment before the call.
 grep '^\s+\w+ = .*\bappend\(\w+,(\s+\w+\[\d+\][\^\-+]\w+\[\d+\],?)+\)$' debug1/test/main/extra_literals.go
 
-# TODO: re-enable once https://github.com/golang/go/issues/47631 is fixed
 # XorSeed obfuscator. Detect type decFunc func(byte) decFunc
-# grep '^\s+type \w+ func\(byte\) \w+$' debug1/test/main/extra_literals.go 
+grep '^\s+type \w+ func\(byte\) \w+$' debug1/test/main/extra_literals.go
 
 # Finally, sanity check that we can build all of std with -literals.
 # Analogous to gogarble.txt.