Rewrite ConstStructBuilder with a mechanism that can cope with splitting and updating constants.
This adds a ConstantBuilder class that deals with incrementally building
an aggregate constant, including support for overwriting
previously-emitted parts of the aggregate with new values.
This fixes a bunch of cases where we used to be unable to reduce a
DesignatedInitUpdateExpr down to an IR constant, and also lays some
groundwork for emission of class constants with [[no_unique_address]]
Differential Revision: https://reviews.llvm.org/D63371