Disable warnings for the generated LLDB wrapper source

Summary:
This source files emits all kind of compiler warnings on different platforms. As the source code
in the file is generated and we therefore can't actually fix the warnings, we might as well disable
them.

Reviewers: aprantl, davide

Reviewed By: davide

Subscribers: davide, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D48096

llvm-svn: 334557