[CMake] Export utility targets to the build/install tree depending on LLVM_BUILD/INSTALL_UTILS

[sgraenitz]

Summary

Allow external projects to import test-related targets like FileCheck, count, not etc. and query binary paths, properties, etc.
This would be useful for LLDB, because it reduces the difference between in-tree vs. standalone builds and simplifies CMake logic.

[beanz]

LGTM

llvm/trunk/cmake/modules/AddLLVM.cmake

if( LLVM_INSTALL_UTILS AND LLVM_BUILD_UTILS )
     install (TARGETS ${name}
       RUNTIME DESTINATION ${LLVM_UTILS_INSTALL_DIR}
       COMPONENT ${name})
     if (NOT LLVM_ENABLE_IDE)
       add_llvm_install_targets(install-${name}
                                DEPENDS ${name}
                                COMPONENT ${name})
     endif()
+    set_property(GLOBAL APPEND PROPERTY LLVM_EXPORTS ${name})
+  elseif( LLVM_BUILD_UTILS )
+    set_property(GLOBAL APPEND PROPERTY LLVM_EXPORTS_BUILDTREE_ONLY ${name})
   endif()
 endmacro(add_llvm_utility name)
 
 macro(add_llvm_fuzzer name)
   cmake_parse_arguments(ARG "" "DUMMY_MAIN" "" ${ARGN})
   if( LLVM_LIB_FUZZING_ENGINE )
     set(LLVM_OPTIONAL_SOURCES ${ARG_DUMMY_MAIN})
     add_llvm_executable(${name} ${ARG_UNPARSED_ARGUMENTS})