[clang-rename] Handle designated initializers.

clang Tooling, and more specifically Refactoring/Rename, have support
code to extract source locations given a Unified Symbol Resolution set.
This support code is used by clang-rename and other tools that might not
be in the tree.

Currently field designated initializer are not supported.
So, renaming S::a to S::b in this code:

  S s = { .a = 10 };

will not extract the field designated initializer for a (the 'a' after the
dot).

This patch adds support for field designated initialized to
RecursiveSymbolVisitor and RenameLocFinder that is used in
createRenameAtomicChanges.

Differential Revision: https://reviews.llvm.org/D100310