If the copy constructor is defined then the copy assignment
must also be defined, but we can use the default here.

v2: Don't inline because the chromium-style errors out on this

Bug: angleproject:4486
Change-Id: Iec0f63342244249da32b455b49a0b89b6558cab9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2690953Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>