[cmake] Make install of stdlib target libraries OPTIONAL

This fixes using --install-swift on macOS when not building all of the
variant stdlibs (e.g. iphonesimulator, etc.).

When we build swift on macOS, by default we build only the macOS stdlib.
The other stdlib variants are still configured and there are targets to
build them if you e.g. run `ninja swift-stdlib-iphonesimulator-x86_64`
manually.  However, we do not separate the _install_ actions.  This
meant that you couldn't install without building all the configured
stdlib variants.

Author: benlangmuir

cmake/modules/AddSwift.cmake

@@ -1985,7 +1985,8 @@ function(add_swift_target_library name)
         swift_install_in_component("${SWIFTLIB_INSTALL_IN_COMPONENT}"
             FILES "${UNIVERSAL_LIBRARY_NAME}"
             DESTINATION "lib${LLVM_LIBDIR_SUFFIX}/${resource_dir}/${resource_dir_sdk_subdir}"
-            PERMISSIONS ${file_permissions})
+            PERMISSIONS ${file_permissions}
+            OPTIONAL)
       endif()
       if(sdk STREQUAL WINDOWS)
         foreach(arch ${SWIFT_SDK_WINDOWS_ARCHITECTURES})
@@ -2032,7 +2033,8 @@ function(add_swift_target_library name)
             PERMISSIONS
               OWNER_READ OWNER_WRITE
               GROUP_READ
-              WORLD_READ)
+              WORLD_READ
+            OPTIONAL)
       endif()
 
       # Add Swift standard library targets as dependencies to the top-level

cmake/modules/SwiftSource.cmake

@@ -349,7 +349,8 @@ function(_compile_swift_files
 
   swift_install_in_component("${SWIFTFILE_INSTALL_IN_COMPONENT}"
     FILES ${module_outputs}
-    DESTINATION "lib${LLVM_LIBDIR_SUFFIX}/swift/${library_subdir}")
+    DESTINATION "lib${LLVM_LIBDIR_SUFFIX}/swift/${library_subdir}"
+    OPTIONAL)
 
   set(line_directive_tool "${SWIFT_SOURCE_DIR}/utils/line-directive")
   set(swift_compiler_tool "${SWIFT_NATIVE_SWIFT_TOOLS_PATH}/swiftc")