Implement DR990 and DR1070. Aggregate initialization initializes uninitialized

Implement DR990 and DR1070. Aggregate initialization initializes uninitialized
elements from {}, rather than value-initializing them. This permits calling an
initializer-list constructor or constructing a std::initializer_list object.
(It would also permit initializing a const reference or rvalue reference if
that weren't explicitly prohibited by other rules.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210091 91177308-0d34-0410-b5e6-96231b3b80d8