Revert "Intel: Add Intel Clang compiler identification"

Revert commit 5c3a93ab88 (Intel: Add Intel Clang compiler
identification, 2020-09-29, v3.19.0-rc1~68^2).  The compiler has already
been released, and is more usable with CMake by pretending to be
upstream Clang than by identifying it as a compiler for which we have
not implemented support.

Issue: #21551

Help/variable/CMAKE_LANG_COMPILER_ID.rst

@@ -26,7 +26,6 @@ include:
   IAR = IAR Systems (iar.com)
   Intel = Intel Compiler (intel.com)
   IntelDPCPP = Intel DPCPP Compiler (intel.com)
-  IntelClang = Intel Clang Compiler (intel.com)
   MSVC = Microsoft Visual Studio (microsoft.com)
   NVIDIA = NVIDIA CUDA Compiler (nvidia.com)
   OpenWatcom = Open Watcom (openwatcom.org)

Modules/CMakeCompilerIdDetection.cmake

@@ -79,7 +79,6 @@ function(compiler_id_detection outvar lang)
       AppleClang
       ARMClang
       IntelDPCPP
-      IntelClang
       Clang
       GNU
       MSVC

Modules/Compiler/IntelClang-DetermineCompiler.cmake

@@ -1,7 +0,0 @@
-
-set(_compiler_id_pp_test "defined(__clang__) && defined(__INTEL_CLANG_COMPILER)")
-
-include("${CMAKE_CURRENT_LIST_DIR}/Clang-DetermineCompilerInternal.cmake")
-
-string(APPEND _compiler_id_version_compute "
-# define @PREFIX@COMPILER_VERSION_TWEAK @MACRO_DEC@(__INTEL_CLANG_COMPILER)")