[dev.typeparams] cmd/compile: add -d=unifiedquirks for quirks mode

Originally, overloading -d=inlfuncswithclosures=0 to enable quirks
mode was convenient because toolstash -cmp doesn't provide a way to
pass different gcflags to the installed vs stashed toolchains. Prior
to unified IR being merged, the stashed toolchain wouldn't know about
or accept any unified-specific flags.

However, this concern is no longer applicable since unified IR has
been merged, and the TestUnifiedCompare test can easily specify
different flag sets for the baseline and experiment build configs.

This CL adds a new -d=unifiedquirks flag to enable quirks mode, so
that it's possible to test unified IR with -d=inlfuncswithclosures=0
without also affecting a bunch of other compilation details.

Change-Id: Id1932f332822622aa8617278e82ec6d1a53b1b46
Reviewed-on: https://go-review.googlesource.com/c/go/+/329733


Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>