Sema: Properly perform lookup when acting on fields for desig inits

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
typo-correction path.

This has the side effect of speeding up semantic handling of the fields
for a designated initializer while simplifying the code at the same
time.

This fixes PR20573.

Differential Revision: http://reviews.llvm.org/D4839