build: honour SWIFT_CLANG_LOCATION

We would previously use `clang-cl` without the explicit path which could
find an alternative `clang-cl` instance.  This manifested as a failure
to build libdispatch as part of the compiler when cross-compiling x64 to
arm64.

Author: compnerd

cmake/modules/Libdispatch.cmake

@@ -14,6 +14,9 @@ if(NOT CMAKE_SYSTEM_NAME STREQUAL Darwin)
           $<TARGET_FILE_DIR:clang>/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
         set(SWIFT_LIBDISPATCH_CXX_COMPILER
           $<TARGET_FILE_DIR:clang>/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
+      elseif(DEFINED SWIFT_CLANG_LOCATION)
+        set(SWIFT_LIBDISPATCH_C_COMPILER ${SWIFT_CLANG_LOCATION}/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
+        set(SWIFT_LIBDISPATCH_CXX_COMPILER ${SWIFT_CLANG_LOCATION}/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
       else()
         set(SWIFT_LIBDISPATCH_C_COMPILER clang-cl${CMAKE_EXECUTABLE_SUFFIX})
         set(SWIFT_LIBDISPATCH_CXX_COMPILER clang-cl${CMAKE_EXECUTABLE_SUFFIX})