Disable LLVM_ENABLE_ASSERTIONS

LLVM_ENABLE_ASSERTIONS is disabled for clang linux and darwin builds.
Windows can not disable LLVM_ENABLE_ASSERTIONS due to
http://b/62787860.

Test: ./build.py
Bug: http://b/62726401, http://b/62787860
Change-Id: I225af0cffea773d4da08ecbe0b1dc530d1da4b19
1 file changed