[cmake] Port the llvm-config option --build-mode to LLVMConfig.cmake via the variable LLVM_BUILD_TYPE.
[cmake] Port the llvm-config option --build-mode to LLVMConfig.cmake via the variable LLVM_BUILD_TYPE.

This is just a small step in the direction of making LLVMConfig.cmake a complete
replacement for llvm-config.

For those unfamiliar, llvm-config --build-mode prints out CMAKE_BUILD_TYPE. Thus
as one can imagine, LLVM_BUILD_TYPE is @CMAKE_BUILD_TYPE@.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@273782 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 0a04f70a4b425044237117dc5bbe462124ad82fb)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm