[clangd] Delete ctor initializers while moving functions out-of-line

Summary:
Currently we only delete function body from declaration, in addition to
that we should also drop ctor initializers.

Unfortunately CXXConstructorDecl doesn't store the location of `:` before
initializers, therefore we make use of token buffer to figure out where to start
deletion.

Fixes https://github.com/clangd/clangd/issues/220

Reviewers: hokein, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71188