[cmake] Add support for external clang/clang-cl

We need an option to pass external host clang location in case
we can't use just-built clang, e.g. when we are building for
different target architecture.

Also, this adds target options for clang-cl, enabling us to
build for arm64 on Windows@amd64.

Author: lxbndr

cmake/modules/AddSwift.cmake

@@ -118,13 +118,19 @@ function(_add_host_variant_c_compile_link_flags name)
     set(DEPLOYMENT_VERSION ${SWIFT_ANDROID_API_LEVEL})
   endif()
 
-  # MSVC, clang-cl, gcc don't understand -target.
-  if(CMAKE_C_COMPILER_ID MATCHES "Clang" AND NOT SWIFT_COMPILER_IS_MSVC_LIKE)
+  # MSVC and gcc don't understand -target.
+  # clang-cl understands --target.
+  if(CMAKE_C_COMPILER_ID MATCHES "Clang")
     get_target_triple(target target_variant "${SWIFT_HOST_VARIANT_SDK}" "${SWIFT_HOST_VARIANT_ARCH}"
       MACCATALYST_BUILD_FLAVOR ""
       DEPLOYMENT_VERSION "${DEPLOYMENT_VERSION}")
-    target_compile_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:-target;${target}>)
-    target_link_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:-target;${target}>)
+    if("${CMAKE_C_COMPILER_FRONTEND_VARIANT}" STREQUAL "MSVC") # clang-cl options
+      target_compile_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:--target=${target}>)
+      target_link_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:--target=${target}>)
+    else()
+      target_compile_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:-target;${target}>)
+      target_link_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:C,CXX,OBJC,OBJCXX>:-target;${target}>)
+    endif()
   endif()
 
   if (CMAKE_Swift_COMPILER)

cmake/modules/SwiftWindowsSupport.cmake

@@ -83,29 +83,30 @@ macro(swift_swap_compiler_if_needed target)
   if(NOT CMAKE_C_COMPILER_ID MATCHES Clang)
     if(CMAKE_SYSTEM_NAME STREQUAL CMAKE_HOST_SYSTEM_NAME AND CMAKE_SYSTEM_PROCESSOR STREQUAL CMAKE_HOST_SYSTEM_PROCESSOR)
       if(SWIFT_BUILT_STANDALONE)
-        get_target_property(CLANG_LOCATION clang LOCATION)
-        get_filename_component(CLANG_LOCATION ${CLANG_LOCATION} DIRECTORY)
+        get_target_property(SWIFT_CLANG_LOCATION clang LOCATION)
+        get_filename_component(SWIFT_CLANG_LOCATION ${SWIFT_CLANG_LOCATION} DIRECTORY)
       else()
-        set(CLANG_LOCATION ${LLVM_RUNTIME_OUTPUT_INTDIR})
+        set(SWIFT_CLANG_LOCATION ${LLVM_RUNTIME_OUTPUT_INTDIR})
       endif()
+    endif()
 
+    if (DEFINED SWIFT_CLANG_LOCATION)
       if("${CMAKE_C_COMPILER_ID}" STREQUAL "MSVC" OR "${CMAKE_C_SIMULATE_ID}" STREQUAL "MSVC")
-        set(CMAKE_C_COMPILER ${CLANG_LOCATION}/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
-        set(CMAKE_CXX_COMPILER ${CLANG_LOCATION}/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
-        set(CMAKE_C_COMPILER_ID Clang)
+        set(CMAKE_C_COMPILER ${SWIFT_CLANG_LOCATION}/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
+        set(CMAKE_CXX_COMPILER ${SWIFT_CLANG_LOCATION}/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
         set(CMAKE_C_SIMULATE_ID MSVC)
         set(CMAKE_C_COMPILER_FRONTEND_VARIANT MSVC)
-        set(CMAKE_CXX_COMPILER_ID Clang)
         set(CMAKE_CXX_SIMULATE_ID MSVC)
         set(CMAKE_CXX_COMPILER_FRONTEND_VARIANT MSVC)
       else()
-        set(CMAKE_C_COMPILER ${CLANG_LOCATION}/clang${CMAKE_EXECUTABLE_SUFFIX})
-        set(CMAKE_CXX_COMPILER ${CLANG_LOCATION}/clang++${CMAKE_EXECUTABLE_SUFFIX})
-        set(CMAKE_C_COMPILER_ID Clang)
-        set(CMAKE_CXX_COMPILER_ID Clang)
+        set(CMAKE_C_COMPILER ${SWIFT_CLANG_LOCATION}/clang${CMAKE_EXECUTABLE_SUFFIX})
+        set(CMAKE_CXX_COMPILER ${SWIFT_CLANG_LOCATION}/clang++${CMAKE_EXECUTABLE_SUFFIX})
       endif()
+      set(CMAKE_C_COMPILER_ID Clang)
+      set(CMAKE_CXX_COMPILER_ID Clang)
+      message(STATUS "C/C++ compiler for ${target} is set to: ${CMAKE_C_COMPILER}")
     else()
-      message(SEND_ERROR "${target} requires a clang based compiler")
+      message(SEND_ERROR "${target} requires a clang based compiler. Please set SWIFT_CLANG_LOCATION.")
     endif()
   endif()
 endmacro()