AIX: Add support for Clang compiler

The platform information module for GNU on AIX can be reused for Clang
on AIX because clang accepts almost all of the same options.

Help/release/dev/aix-clang.rst

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+aix-clang
+---------
+
+* The Clang compiler is now supported on AIX.

Modules/Platform/AIX-Clang-C.cmake

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+include(Platform/AIX-GNU-C)

Modules/Platform/AIX-Clang-CXX.cmake

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+include(Platform/AIX-GNU-CXX)