`printf()` length modifier support for builtin DFP types

[tahonermann]

ISO/IEC TS 18661-2:2015 and C23 add new H, D, and DD length modifiers to printf() and friends to support formatting of builtin DFP types. Clang diagnoses incorrect format specifiers and will require updates to handle these new length modifiers. See clang/lib/AST/PrintfFormatString.cpp.