Move undefined-func-template to wont-fix warnings.

Move undefined-func-template to wont-fix warnings.

The warning looks to helpfully pre-warn about possible link failures, but it's warning exclusively in places where we're doing things right.

The worst that happens ignoring this warning is a missing-symbol linker error.

I've taken the opportunity to batch in a few other de-escalations:
  - Wconditional-uninitialized is done better by MSAN
  - It'll take some work to dig Wformat-literal out of our shader compiler, but nothing looks unsafe
  - Most of Wshift-sign-overflow is 0xff << 24.  Don't want to ban that.
  - Wdeprecated is mostly warning about features C++11 technically deprecated that might be removed in later releases.  Punt!
  - Wcovered-switch-default is pretty much the opposite of what we want.
  - Wshadow is triggering too often to fix quickly.  Probably mostly false positives.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2620

Change-Id: I20a85a77d2e19ed05a536b23037bd988350f821e
Reviewed-on: https://skia-review.googlesource.com/2620
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>