CMakeCCompilerId: Fix C standard detection in Clang and IntelLLVM MSVC mode

Clang does not define `__STDC__` if in MSVC compatibility mode, but does
define `__STDC_VERSION__`.  Avoid the fallback for this combination.

This backports commit 7596d8b951 (CMakeCCompilerId: Fix C standard
detection in Clang MSVC mode, 2021-02-07, v3.21.0-rc1~587^2~14) to the
3.20 release series.  This is needed since commit 5115dd1e2c (IntelLLVM:
Fix C/C++ standard level flags on Windows, 2021-07-07, v3.21.0-rc3~7^2^2)
now that we activate C/C++ standard level logic for IntelLLVM when
targeting the MSVC ABI.

Modules/CMakeCCompilerId.c.in

@@ -33,9 +33,8 @@ char const *info_cray = "INFO" ":" "compiler_wrapper[CrayPrgEnv]";
 @CMAKE_C_COMPILER_ID_PLATFORM_CONTENT@
 @CMAKE_C_COMPILER_ID_ERROR_FOR_TEST@
 
-#if !defined(__STDC__)
-# if (defined(_MSC_VER) && !defined(__clang__)) \
-  || (defined(__ibmxl__) || defined(__IBMC__))
+#if !defined(__STDC__) && !defined(__clang__)
+# if defined(_MSC_VER) || defined(__ibmxl__) || defined(__IBMC__)
 #  define C_DIALECT "90"
 # else
 #  define C_DIALECT