Clean up trivial default constructors now.

hasTrivialDefaultConstructor() really really means it now.

Also implement a fun standards bug regarding aggregates. Doug, if you'd
like, I can un-implement that bug if you think it is truly a defect.

The bug is that non-special-member constructors are never considered
user-provided, so the following is an aggregate:

struct foo {
  foo(int);
};

It's kind of bad, but the solution isn't obvious - should

struct foo {
  foo (int) = delete;
};

be an aggregate or not?

Lastly, add a missing initialization to FunctionDecl.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@131101 91177308-0d34-0410-b5e6-96231b3b80d8