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

[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@.

llvm-svn: 273782