Id generation warnings in C++ declarations with expressions

[jakobandersen]

(See also breathe-doc/breathe#218)
Declarations that contain expressions, e.g.,

.. cpp:function:: template <typename F> \
                  void allow(F* f, typename functionSignature<F, B, G != 1>::tt_type tt)

may issue warnings about failed id generation, e.g.,

 WARNING: Index id generation for C++ object "template<typename F> void allow(F *f, typename functionSignature<F, B, G!=1>::tt_type tt)" failed, please report as bug (id=_CPPv2I0E5allowP1FN17functionSignatureI1F1BXG!=1EE7tt_typeE).

This is because expressions are parsed in a very naive way, such that the expressions them self are used as part of the id. Two different declarations may therefore incidentally get the same id.

What characters are actually allowed in ids? Are writers or domains responsible for doing proper escaping? (maybe @shimizukawa knows?) If the writers are, then maybe the warning should only be in verbose output.

[jakobandersen]

Expressions are now properly parsed.