Tests: Fix RunCMake.try_compile C/CXX standards with IntelLLVM MSVC mode

Since commit 5115dd1e (IntelLLVM: Fix C/C++ standard level flags on
Windows, 2021-07-07, v3.21.0-rc3~7^2^2) we activate C/C++ standard level
logic for IntelLLVM when targeting the MSVC ABI.  Update the
`RunCMake.try_compile` test to be aware of this even when CMake is
itself configured by an older CMake that does not know this.

Tests/RunCMake/CMakeLists.txt

@@ -420,6 +420,19 @@ function(add_RunCMake_test_try_compile)
       set(CMAKE_C_STANDARD_DEFAULT "")
     endif()
   endif()
+  if(CMAKE_VERSION VERSION_LESS 3.20.6 AND "x${CMAKE_C_COMPILER_ID}" STREQUAL "xIntelLLVM" AND "x${CMAKE_C_SIMULATE_ID}" STREQUAL "xMSVC")
+    # Older CMake versions accidentally set the default standards to empty when
+    # IntelLLVM targets the MSVC ABI, thus not activating standard selection.
+    # Approximate the logic from IntelLLVM-{C,CXX}.cmake.
+    if(DEFINED CMAKE_C_STANDARD_DEFAULT AND "${CMAKE_C_STANDARD_DEFAULT}" STREQUAL "")
+      # FIXME: The compiler actually defaults to C17, but
+      # CMake does not yet model or detect that standard.
+      set(CMAKE_C_STANDARD_DEFAULT 11)
+    endif()
+    if(DEFINED CMAKE_CXX_STANDARD_DEFAULT AND "${CMAKE_CXX_STANDARD_DEFAULT}" STREQUAL "")
+      set(CMAKE_CXX_STANDARD_DEFAULT 14)
+    endif()
+  endif()
   foreach(var
       CMAKE_SYSTEM_NAME
       CMAKE_C_COMPILER_ID