[clang] Enable code completion of designated initializers in Compound Literal Expressions

PreferedType were not set when parsing compound literals, hence
designated initializers were not available as code completion suggestions.

This patch sets the preferedtype to parsed type for the following initializer
list.

Fixes https://github.com/clangd/clangd/issues/142.

Differential Revision: https://reviews.llvm.org/D92370