Add a target environment for CoreCLR.

Add a target environment for CoreCLR.

Although targeting CoreCLR is similar to targeting MSVC, there are
certain important differences that the backend must be aware of
(e.g. differences in stack probes, EH, and library calls).

Differential Revision: http://reviews.llvm.org/D11012

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245115 91177308-0d34-0410-b5e6-96231b3b80d8