[lldb/DWARF] Add a utility function for (forceful) completion of types

Summary:
Unify the code for requiring a complete type and move it into a single
place. The only functional change is that the "cannot start a definition
of an incomplete type" is upgrated from a runtime error/warning to an
lldbassert. An plain assert might also be fine, since (AFAICT) this can
only happen in case of a programmer error.

Reviewers: teemperor, aprantl, shafik

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D83199