As reported in #113, recent versions of Clang complain about
ambiguous overloads for some comparison operator instantiations,
especially if the deduced template type is a GenericValue.
Add an explicit, non-templated version for now (which is a better
match).
This only solves part of the problem, as comparisons between
* GenericValue & GenericDocument
* GenericValue with different SourceAllocator types
will still cause ambiguities.
a9add7bd2a