[CMake][AIX] Fixing AIX rpath

Recent commit 8f833f8 modified the installation rpath and did not set `BUILD_WITH_INSTALL_RPATH` correctly on AIX, which led to installation failures on AIX. This patch sets `BUILD_WITH_INSTALL_RPATH` on AIX to fix the installation failures.

Reviewed By: buttaface, daltenty

Differential Revision: https://reviews.llvm.org/D148866

(cherry picked from commit 887828d)

Author: Qiongsi Wu

llvm/cmake/modules/AddLLVM.cmake

@@ -2318,8 +2318,7 @@ function(llvm_setup_rpath name)
     # Since BUILD_SHARED_LIBS is only recommended for use by developers,
     # hardcode the rpath to build/install lib dir first in this mode.
     # FIXME: update this when there is better solution.
-    set(_build_rpath "${LLVM_LIBRARY_OUTPUT_INTDIR}" ${extra_libdir})
-    set(_install_rpath "${CMAKE_INSTALL_PREFIX}/lib${LLVM_LIBDIR_SUFFIX}")
+    set(_install_rpath "${LLVM_LIBRARY_OUTPUT_INTDIR}" "${CMAKE_INSTALL_PREFIX}/lib${LLVM_LIBDIR_SUFFIX}" ${extra_libdir})
   elseif(UNIX)
     set(_build_rpath "\$ORIGIN/../lib${LLVM_LIBDIR_SUFFIX}" ${extra_libdir})
     set(_install_rpath "\$ORIGIN/../lib${LLVM_LIBDIR_SUFFIX}")
@@ -2337,9 +2336,11 @@ function(llvm_setup_rpath name)
   endif()
 
   # Enable BUILD_WITH_INSTALL_RPATH unless CMAKE_BUILD_RPATH is set and not
-  # building for macOS or Windows, as those two platforms seemingly require it.
+  # building for macOS or AIX, as those platforms seemingly require it.
+  # On AIX, the tool chain doesn't support modifying rpaths/libpaths for XCOFF
+  # on install at the moment, so BUILD_WITH_INSTALL_RPATH is required.
   if("${CMAKE_BUILD_RPATH}" STREQUAL "")
-    if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin|Windows")
+    if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin|AIX")
       set_property(TARGET ${name} PROPERTY BUILD_WITH_INSTALL_RPATH ON)
     else()
       set_property(TARGET ${name} APPEND PROPERTY BUILD_RPATH "${_build_rpath}")