[CodeCompletion] (mostly) fix completion in incomplete C++ ctor initializers.

C++ member function bodies (including ctor initializers) are first captured
into a buffer and then parsed after the class is complete. (This allows
members to be referenced even if declared later).

When the boundary of the function body cannot be established, its buffer is
discarded and late-parsing never happens (it would surely fail).
For code completion this is the wrong tradeoff: the point of the parse is to
generate completions as a side-effect.
Today, when the ctor body wasn't typed yet there are no init list completions.
With this patch we parse such an init-list if it contains the completion point.

There's one caveat: the parser has to decide where to resume parsing members
after a broken init list. Often the first clear recovery point is *after* the
next member, so that member is missing from completion/signature help etc. e.g.
  struct S {
    S() m  //<- completion here
    int maaa;
    int mbbb;
  }
Here "int maaa;" is treated as part of the init list, so "maaa" is not available
as a completion. Maybe in future indentation can be used to recognize that
this is a separate member, not part of the init list.

Differential Revision: https://reviews.llvm.org/D116294

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