Disable warnings triggered in Clang r271374

http://b/28149048
http://b/29823425

Disable -Wconstant-conversion and -Wundefined-var-template.  The second
bug above tracks that these warnings get reenabled.

Test: Tested build, boot and common usage for Arm, Arm64, x86, x86_64,
Mips images in AOSP and internal branch.

Change-Id: Iea20cf6b5dbec3247b55cf8130f88202e786e367