clang: Work around toolchain file use of internal CMake variables

Since commit d44c0db (clang: setup correct configuration in gnu mode,
2019-02-20, v3.15.0-rc1~41^2~5) our Clang compiler information modules
need the `CMAKE_<LANG>_COMPILER_FRONTEND_VARIANT` variable that we
compute during compiler detection.  However, some existing toolchain
files set our internal `CMAKE_<LANG>_COMPILER_ID_RUN` variables and
block that detection, but do not set the new frontend variant variable.
Help them out by setting `CMAKE_<LANG>_COMPILER_FRONTEND_VARIANT` anyway.

Fixes: #19515, #19539

Author: bradking

Modules/CMakeDetermineCCompiler.cmake

@@ -124,6 +124,22 @@ if(NOT CMAKE_C_COMPILER_ID_RUN)
   elseif(CMAKE_C_PLATFORM_ID MATCHES "Cygwin")
     set(CMAKE_COMPILER_IS_CYGWIN 1)
   endif()
+else()
+  if(NOT DEFINED CMAKE_C_COMPILER_FRONTEND_VARIANT)
+    # Some toolchain files set our internal CMAKE_C_COMPILER_ID_RUN
+    # variable but are not aware of CMAKE_C_COMPILER_FRONTEND_VARIANT.
+    # They pre-date our support for the GNU-like variant targeting the
+    # MSVC ABI so we do not consider that here.
+    if(CMAKE_C_COMPILER_ID STREQUAL "Clang")
+      if("x${CMAKE_C_SIMULATE_ID}" STREQUAL "xMSVC")
+        set(CMAKE_C_COMPILER_FRONTEND_VARIANT "MSVC")
+      else()
+        set(CMAKE_C_COMPILER_FRONTEND_VARIANT "GNU")
+      endif()
+    else()
+      set(CMAKE_C_COMPILER_FRONTEND_VARIANT "")
+    endif()
+  endif()
 endif()
 
 if (NOT _CMAKE_TOOLCHAIN_LOCATION)

Modules/CMakeDetermineCXXCompiler.cmake

@@ -119,6 +119,22 @@ if(NOT CMAKE_CXX_COMPILER_ID_RUN)
   elseif(CMAKE_CXX_PLATFORM_ID MATCHES "Cygwin")
     set(CMAKE_COMPILER_IS_CYGWIN 1)
   endif()
+else()
+  if(NOT DEFINED CMAKE_CXX_COMPILER_FRONTEND_VARIANT)
+    # Some toolchain files set our internal CMAKE_CXX_COMPILER_ID_RUN
+    # variable but are not aware of CMAKE_CXX_COMPILER_FRONTEND_VARIANT.
+    # They pre-date our support for the GNU-like variant targeting the
+    # MSVC ABI so we do not consider that here.
+    if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+      if("x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC")
+        set(CMAKE_CXX_COMPILER_FRONTEND_VARIANT "MSVC")
+      else()
+        set(CMAKE_CXX_COMPILER_FRONTEND_VARIANT "GNU")
+      endif()
+    else()
+      set(CMAKE_CXX_COMPILER_FRONTEND_VARIANT "")
+    endif()
+  endif()
 endif()
 
 if (NOT _CMAKE_TOOLCHAIN_LOCATION)