Rename llvm::array_lengthof into llvm::size to match std::size from C++17
As a conquence move llvm::array_lengthof from STLExtras.h to STLForwardCompat.h (which is included by STLExtras.h so no build breakage expected).
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As a conquence move llvm::array_lengthof from STLExtras.h to STLForwardCompat.h (which is included by STLExtras.h so no build breakage expected).