[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: https://llvm.org/svn/llvm-project/llvm/trunk@273782 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 0a04f70)

Author: gottesmm

cmake/modules/LLVMConfig.cmake.in

@@ -7,6 +7,8 @@ set(LLVM_VERSION_MINOR @LLVM_VERSION_MINOR@)
 set(LLVM_VERSION_PATCH @LLVM_VERSION_PATCH@)
 set(LLVM_PACKAGE_VERSION @PACKAGE_VERSION@)
 
+set(LLVM_BUILD_TYPE @CMAKE_BUILD_TYPE@)
+
 set(LLVM_COMMON_DEPENDS @LLVM_COMMON_DEPENDS@)
 
 set(LLVM_AVAILABLE_LIBS @LLVM_AVAILABLE_LIBS@)