build/soong sets default clang version to 'clang-copperhead':

dkrishnamurthy@CLI-4766 ~/stub/git/workspace/CopperheadOS
[2476]$ ls -l prebuilts/clang/host/darwin-x86/clang-copperhead
lrwxr-xr-x  1 dkrishnamurthy  2032920826  13 Apr 13 20:30 prebuilts/clang/host/darwin-x86/clang-copperhead@ -> clang-4691093

Bison crashes, use Bison from homebrew instead:

dkrishnamurthy@CLI-4766 ~/stub/git/workspace/CopperheadOS
[2234]$ ls -l prebuilts/misc/darwin-x86/bison/bison
lrwxr-xr-x  1 dkrishnamurthy  2032920826  30 Apr 12 19:43 prebuilts/misc/darwin-x86/bison/bison@ -> /usr/local/opt/bison/bin/bison
dkrishnamurthy@CLI-4766 ~/stub/git/workspace/CopperheadOS
[2235]$ ls -l external/bison/data
lrwxr-xr-x  1 dkrishnamurthy  2032920826  43 Apr 12 19:52 external/bison/data@ -> /usr/local/Cellar/bison/3.0.4_1/share/bison