Clang used a custom implementation of lookup when handling designated
initializers. The custom code was not particularly optimized and relied
on standard lookup for typo-correction anyway.
This custom code has to go, it doesn't properly support MSVC-style
anonymous structs embedded inside other records; replace it with the
This has the side effect of speeding up semantic handling of the fields
for a designated initializer while simplifying the code at the same
This fixes PR20573.