doxygen: fix and improve macro expansion configuration options

[eramongodb]

Workaround doxgyen/doxygen#11495 and doxygen/doxygen#11496 which are affecting CXX-2745 API documentation by tidying up the preprocessor-related Doxygen configuration options following #1301. Since all relevant macro definitions are now visible, they can be expanded as-is with MACRO_EXPANSION = YES without using EXPAND_AS_DEFINED.

EXPAND_ONLY_PREDEF = YES and SKIP_FUNCTION_MACROS = YES are no longer required to guard against the attempted expansion of complex macros, as all problematic expansions are already being guarded with \cond DOXYGEN_DISABLE (these options do not help us avoid these issues) and we do not parse non-header files (where most other macros, including those from external libraries, are used).

Due to the complexity of export macro definitions confusing the Doxygen preprocessor (notably their expansion into __attribute__ and __declspec specifiers), PREDEFINED is used to replace the definitions of *_ABI_EXPORT, *_ABI_CDECL, and *_ABI_EXPORT_CDECL with simpler alternatives. All three must be predefined to fully avoid preprocessor and parser failures.

Given the *_ABI_EXPORT= predefinitions, the *_ABI_EXPORT macro is never defined in code due to the #ifndef guard in the generated export header. A Doxygen preprocessor branch which unconditionally defines the export macro (as a stub: ...) is needed to satisfy the \def special command's requirement that the documented macro is defined in code prior to the doc comment.