Debugger: always prepend '::' to types when constructing expressions

Traditionally, this would have been exactly the wrong thing to do as the
colons confused gdb's command line parser. Now that the general C++
awareness has improved, the parser accepts :: but gets confused by name
lookups, see http://sourceware.org/bugzilla/show_bug.cgi?id=15008



Change-Id: I01e46643a5179ecef26fcc11eab7bab84c359ec6
Reviewed-by: hjk <qthjk@ovi.com>