Have lldb_assert accept bool expressions

Summary:
This changes lldb_assert to accept bool expressions as the parameter, this is because some
objects (such as std::shared_ptr) are convertible to bool, but are not convertible to int, which
leads to surprising errors.

Reviewers: granata.enrico, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D9565

llvm-svn: 236819