[Clang] Fix the location of default init expressions

Default member initializations constructed from
a parenthesized aggregate initialization should be constructed
at the location of the left paren, to be consistent with
brace initialization.
Otherwise we get diagmostics and source_location in the wrong places.

Fixes #63903

Reviewed By: aaron.ballman, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D155573