Move LLVM_ENABLE_ABI_BREAKING_CHECKS variables to their own file

so that you don't have to link Error.o and all of its dependencies.

In more detail: global initializers in Error.o can't be elided with
-ffunction-sections/-gc-sections since they always need to be run
causing a fairly significant binary bloat if all you want is the
ABI breaking checks code.

Differential Revision: https://reviews.llvm.org/D67387

llvm-svn: 371561