Improve handling of base initializers. We now parse initializers in out of line decls, such as:

class C {
    C() { }
    
    int a;
};

C::C() : a(10) { }

We also diagnose when initializers are used on declarations that aren't constructors:

t.cpp:1:10: error: only constructors take base initializers
void f() : a(10) { }
         ^

Doug and/or Sebastian: I'd appreciate a review, especially the nested-name-spec test results (from the looks of it we now match gcc in that test.)

llvm-svn: 67672