DebugInfo: Correctly identify the location of C++ member initializer list elements

This particularly helps the fidelity of ASan reports (which can occur
even in these examples - if, for example, one uses placement new over a
buffer of insufficient size - now ASan will correctly identify which
member's initialization went over the end of the buffer).

This doesn't cover all types of members - more coming.

llvm-svn: 223726