mangling: Remove FB_DATATYPE_ARRAY (unused since 831f0c34)

(FB's bydesc parameters aren't mangled as C++ array parameters anymore
since 831f0c34 - they're completely different things anyways)
mangling: Remove unused dtype variable
mangling: Add clarifying comment to hAbbrevGet()
mangling: Don't use astGetFullType() macro with FB_MANGLEABBR nodes

(it should only be used with ASTNODEs)
typeDump(): Dump reference flag (used during mangling)
typeDump(): Show namespace name
mangling: Fix nested namespace abbreviations (98e12210 regression)
mangling: Fix FBSTRING abbreviation (a809d7a8 regression)
tests: Replace undeclared identifier FALSE with 0 in -lang qb tests

It only worked previously because of implicit variable declarations.
mangling: Refactor symbMangleType() to clarify how it works
mangling: Fix [U]INTEGER abbreviation mangling issue on win64