PR10572: Allow duplicate function names in a CU

PR10572: Allow duplicate function names in a CU

We can't assume that a given function name will only appear once in a
CU.  In C++, two functions may have the same name in different classes
or namespaces, or even in the same scope with overloaded parameters.
Even in C, the compiler may generate multiple copies of a single
function with different optimizations.

We now use a multimap for function names, so we shouldn't miss any.

* dwflpp.h (cu_type_cache_t, mod_cu_type_cache_t): New typedef to keep a
  normal map for the global_alias_cache.
  (cu_function_cache_t): Use a multimap for function names.
* dwflpp.cxx (dwflpp::iterate_over_functions): Walk over the range of
  exactly-matching functions.
* tapsets.cxx (query_dwarf_func): Don't abort after seeing an exact
  match -- there could be more to come.