gdb/c++: Detect ambiguous variables in imported namespaces

gdb/c++: Detect ambiguous variables in imported namespaces

When running gdb.cp/nsusing.cc and stopping at line 17, we can ask GDB
to print x and get a compiler-dependent answer. Using gcc 12.2.1, GDB
will print M::x, and using clang 16.0.0 prints N::x. Not only is this
behavior confusing to users, it is also not consistent with compiler
behaviors, which would warn that using x is ambiguous at this point.

This commit makes GDB behavior consistent with compilers. it achieves
this by making it so instead of exiting early when finding any symbol
with the correct name, GDB continues searching through all include
directives, storing all matching symbols in a relational map betwen the
mangled name and the found symbols.

If the resulting map has more than one entry, GDB says that the
reference is ambiguous and lists all possibilities. Otherwise it returns
the block_symbol structure for the desired symbol, or an empty struct if
nothing was found.

The commit also changes gdb.cp/nsusing.exp to test the ambiguous
detection.