[clangd] Make use of SourceOrder to find first initializer in DefineOutline

Summary:
Constructors can have implicit initializers, this was crashing define
outline. Make sure we find the first "written" ctor initializer to figure out
`:` location.

Fixes https://github.com/clangd/clangd/issues/400

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80521