[analyzer] MoveChecker: Add an option to suppress warnings on locals.

Re-using a moved-from local variable is most likely a bug because there's
rarely a good motivation for not introducing a separate variable instead.
We plan to keep emitting such warnings by default.

Introduce a flag that allows disabling warnings on local variables that are
not of a known move-unsafe type. If it doesn't work out as we expected,
we'll just flip the flag.

We still warn on move-unsafe objects and unsafe operations on known move-safe
objects.

Differential Revision: https://reviews.llvm.org/D55730

llvm-svn: 349327