llvm_gtest: install to appropriate locations.

[dtzSiFive]

cc @tstellar , can you take a look? Thanks!

[tstellar]

LGTM.

third-party/unittest/CMakeLists.txt

 
 if (NOT LLVM_ENABLE_THREADS)
   target_compile_definitions(llvm_gtest PUBLIC GTEST_HAS_PTHREAD=0)
 endif ()
 
 target_include_directories(llvm_gtest
   PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/googletest/include>
          $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/googlemock/include>
-         $<INSTALL_INTERFACE:include/llvm-gtest/>
-         $<INSTALL_INTERFACE:include/llvm-gmock/>
+         $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/llvm-gtest/>
+         $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/llvm-gmock/>
   PRIVATE googletest googlemock
   )
 
 add_subdirectory(UnitTestMain)
 
 if (LLVM_INSTALL_GTEST)
   install(TARGETS llvm_gtest llvm_gtest_main LLVMTestingSupport LLVMTestingAnnotations
-    ARCHIVE DESTINATION "lib${LLVM_LIBDIR_SUFFIX}" COMPONENT llvm_gtest)
-  install(DIRECTORY googletest/include/gtest/ DESTINATION include/llvm-gtest/gtest/ COMPONENT llvm_gtest)
-  install(DIRECTORY googlemock/include/gmock/ DESTINATION include/llvm-gmock/gmock/ COMPONENT llvm_gtest)
+    ARCHIVE DESTINATION "${CMAKE_INSTALL_LIBDIR}${LLVM_LIBDIR_SUFFIX}" COMPONENT llvm_gtest)
+  install(DIRECTORY googletest/include/gtest/ DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/llvm-gtest/gtest/" COMPONENT llvm_gtest)
+  install(DIRECTORY googlemock/include/gmock/ DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/llvm-gmock/gmock/" COMPONENT llvm_gtest)
 endif()
 
 # When LLVM_LINK_LLVM_DYLIB is enabled, libLLVM.so is added to the interface
 # link libraries for gtest and gtest_main.  This means that any target, like
 # unittests for example, that links against gtest will be forced to link
 # against libLLVM.so.  In some cases we may want to statically unittests if they
 # need access to symbols that are marked private in libLLVM.so.  The only
 # way we can make this work is to remove libLLVM.so from the list of interface