[clangd] Add designator inlay hints for initializer lists.

[clangd] Add designator inlay hints for initializer lists.

These make the init lists appear as if designated initialization was used.

Example:
  ExpectedHint{"param: ", "arg"}
becomes
  ExpectedHint{.Label="param: ", .RangeName="arg"}

Differential Revision: https://reviews.llvm.org/D116786