[lldb-mi] Use size_t where appropriate.

Summary:
Many places should have been using size_t rather than MIuint or
MIint. This is particularly true for code that uses std::string::find(),
std::string::rfind(), std::string::size(), and related methods.

Reviewers: abidh, ki.stfu, domipheus

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D10931

llvm-svn: 241360