Get rid of the C-string parameter in DoExecute

Summary:
This patch gets rid of the C-string parameter in the RawCommandObject::DoExecute function,
making the code simpler and less memory unsafe.

There seems to be a assumption in some command objects that this parameter could be a nullptr,
but from what I can see the rest of the API doesn't actually allow this (and other command
objects and related code pieces dereference this parameter without any checks).

Especially CommandObjectRegexCommand has error handling code for a nullptr that is now gone.

Reviewers: davide, jingham, teemperor

Reviewed By: teemperor

Subscribers: jingham, lldb-commits

Differential Revision: https://reviews.llvm.org/D49207

llvm-svn: 336955