Made a stab at has_default_constructor. Got it mostly working for g++-4.0,...

Made a stab at has_default_constructor.  Got it mostly working for g++-4.0, but only works for scalar types on clang.  Ultimately this needs a compiler-supported is_constructible which clang is missing, and won't be able to use until it gets variadic templates.

llvm-svn: 113225