Silence some MSVC warnings.

Summary:
Just `assert("string" && false)` instead of `assert("string" == NULL)`.

This avoid errors like

    [...]\Core\SourceManager.cpp(647): warning C4130: '==' : logical operation on address of string constant

Reviewers: zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12388

llvm-svn: 246123