[CodeCompletion] Signature help for braced constructor calls

Implementation is based on the "expected type" as used for
designated-initializers in braced init lists. This means it can deduce the type
in some cases where it's not written:

  void foo(Widget);
  foo({ /*help here*/ });

Only basic constructor calls are in scope of this patch, excluded are:
 - aggregate initialization (no help is offered for aggregates)
 - initializer_list initialization (no help is offered for these constructors)

Fixes https://github.com/clangd/clangd/issues/306

Differential Revision: https://reviews.llvm.org/D116317