IntelLLVM: Add special case for ifx 2021.1 version extraction

The ifx beta versions forgot to define `__INTEL_LLVM_COMPILER`, and
instead define `__INTEL_COMPILER == 201900`.  Add a special case.

Issue: #22120

Help/release/3.20.rst

@@ -47,11 +47,7 @@ Compilers
   * The ``icx``/``icpx`` C/C++ compilers on Linux, and the ``icx``
     C/C++ compiler on Windows, are fully supported as of oneAPI 2021.1.
 
-  * The ``ifx`` Fortran compiler on Linux is partially supported.
-    As of oneAPI 2021.1, ``ifx`` does not define several identification
-    macros, so CMake identifies it as the classic ``Intel`` compiler.
-    This works in many cases because ``ifx`` accepts the same command line
-    parameters as ``ifort``.  A future version of oneAPI may fix this.
+  * The ``ifx`` Fortran compiler on Linux is supported as of oneAPI 2021.1.
 
   * The ``ifx`` Fortran compiler on Windows is not yet supported.
 
@@ -359,3 +355,10 @@ Changes made since CMake 3.20.0 include the following.
   as having major version 2021.  CMake 3.20.1 and below were not aware of a
   change to the identification macro version scheme made by Intel starting
   in version 2021, and detected the version as 20.2.
+
+* The Intel oneAPI Fortran compiler is now identified as ``IntelLLVM``.
+  The oneAPI 2021.1 Fortran compiler is missing an identification macro,
+  so CMake 3.20.1 and below identified it as ``Intel``.  CMake now has
+  a special case to recognize oneAPI 2021.1 Fortran as ``IntelLLVM``.
+  The oneAPI 2021.2 Fortran compiler defines the proper identification
+  macro and so is identified as ``IntelLLVM`` by all CMake 3.20 versions.

Modules/CMakeFortranCompilerId.F.in

@@ -36,6 +36,14 @@
 #  define COMPILER_VERSION_MINOR DEC(__INTEL_LLVM_COMPILER/100 % 100)
 #  define COMPILER_VERSION_PATCH DEC(__INTEL_LLVM_COMPILER     % 100)
 # endif
+#elif defined(__INTEL_COMPILER) && __INTEL_COMPILER == 201900
+        PRINT *, 'INFO:compiler[IntelLLVM]'
+! ifx 2021.1 forgot to define __INTEL_LLVM_COMPILER.
+! Instead it defines __INTEL_COMPILER == 201900.
+# define COMPILER_VERSION_MAJOR DEC(2021)
+# define COMPILER_VERSION_MINOR DEC(1)
+# define COMPILER_VERSION_PATCH DEC(__INTEL_COMPILER_UPDATE)
+# define COMPILER_VERSION_TWEAK DEC(__INTEL_COMPILER_BUILD_DATE)
 #elif defined(__INTEL_COMPILER) || defined(__ICC)
         PRINT *, 'INFO:compiler[Intel]'
 ! __INTEL_COMPILER = VRP prior to 2021, and then VVVV for 2021 and later.