[CMake] Export CMAKE_CONFIGURATION_TYPES for the LLVM build-tree

Summary: Useful info for standalone builds of subprojects. If a multi-configuration generator was used for the provided LLVM build-tree, standalone builds should consider actual subdirectories per configuration in `find_program()` (e.g. looking for `llvm-lit` or `llvm-tblgen`).

Reviewers: labath, beanz, mgorny

Subscribers: lldb-commits, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62878

llvm-svn: 362588