Reland "[clang] Fix CTAD for aggregates for nested template classes" (#78670)

Reland of #78387

Use the template pattern in determining whether to synthesize the
aggregate deduction guide, and update
DeclareImplicitDeductionGuideFromInitList to substitute outer template
arguments.

The tests in the original patch made an assumption about the size of a
pointer type, and this led to them failing on targets with 32-bit
pointers. The tests have been updated to not depend on the size of any
type. This only requires updates to the test file, no functionality has
otherwise changed between this and the original patch.