Disable warnings triggered in Clang r271374

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

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

Disable -Wexpansion-to-defined and -Wnull-dereference.  The second bug
above tracks that these warnings get re-enabled.

Change-Id: Ic23f6b51d3f6913fce2b423a15847e8698bab830
1 file changed