Clang: Split Compiler/Clang* modules out from GNU (#13550)

While Clang presents an almost identical interface to GNU there will be
some differences.  Split the compiler information modules to allow
separate rules for Clang.  Start by loading the GNU rules but leave a
place to add Clang-specific information.

Modules/Compiler/Clang-ASM.cmake

@@ -1 +1,5 @@
-include(Compiler/GNU-ASM)
+include(Compiler/Clang)
+
+set(CMAKE_ASM_SOURCE_FILE_EXTENSIONS s;S;asm)
+
+__compiler_clang(ASM)

Modules/Compiler/Clang-C.cmake

@@ -1 +1,2 @@
-include(Compiler/GNU-C)
+include(Compiler/Clang)
+__compiler_clang(C)

Modules/Compiler/Clang-CXX.cmake

@@ -1 +1,2 @@
-include(Compiler/GNU-CXX)
+include(Compiler/Clang)
+__compiler_clang(CXX)

Modules/Compiler/Clang.cmake

@@ -0,0 +1,25 @@
+
+#=============================================================================
+# Copyright 2002-2012 Kitware, Inc.
+#
+# Distributed under the OSI-approved BSD License (the "License");
+# see accompanying file Copyright.txt for details.
+#
+# This software is distributed WITHOUT ANY WARRANTY; without even the
+# implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+# See the License for more information.
+#=============================================================================
+# (To distribute this file outside of CMake, substitute the full
+#  License text for the above reference.)
+
+# This module is shared by multiple languages; use include blocker.
+if(__COMPILER_CLANG)
+  return()
+endif()
+set(__COMPILER_CLANG 1)
+
+include(Compiler/GNU)
+
+macro(__compiler_clang lang)
+  __compiler_gnu(${lang})
+endmacro()