Introduces cmake option `LLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING`

Introduces cmake option `LLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING`

Summary:
We recently introduced a feature that enforce at link-time that the
LLVM headers used by a clients are matching the ABI setting of the
LLVM library linked to.

However for clients that are using only headers from ADT and promise
they won't call into LLVM, this is forcing to link libSupport. This
new flag is intended to provide a way to configure LLVM with this
promise for such client.

Reviewers: bob.wilson, compnerd

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D27432

llvm-svn: 288754