Fix two 'variable is used uninitialised' warnings.  Change assert to llvm_unreachable.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@184334 91177308-0d34-0410-b5e6-96231b3b80d8
The 'register' keyword is now deprecated in C++11, so ignore the warning when compiling lldb with clang since python headers commonly use the keyword.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@184335 91177308-0d34-0410-b5e6-96231b3b80d8