go/types, types2: factor out code for type checking composite literals

go/types, types2: factor out code for type checking composite literals

Move code into separate function in separate file.
Replace "goto Error" statements with "x.mode = invalid; return".
No other semantic changes.

Change-Id: I2d5e858e8df3dc1011fa79cdac3db9d3e7b1dfe5
Reviewed-on: https://go-review.googlesource.com/c/go/+/610556
Reviewed-by: Tim King <taking@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>