This patch to the Go frontend by Cherry Zhang avoids a copy for a
string([]byte) conversion used in string concatenation.  If a
string([]byte) conversion is used immediately in a string
concatenation, we don't need to copy the backing store of the byte
slice, as the runtime function doesn't hold any reference to it.
Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian

2019-06-18  Cherry Zhang  <cherryyz@google.com>

* go.dg/concatstring.go: New test.