Model list initialization more directly; fixes an assert with coverage mapping

Instead of using the validity of a brace's source location as a flag
for list initialization, this now uses a PointerIntPair to model it so
we do not increase the size of the AST node to track this information.
This allows us to retain the valid source location information, which
fixes the coverage assertion.

Fixes https://github.com/llvm/llvm-project/issues/62105
Differential Revision: https://reviews.llvm.org/D148245