[lldb-mi] Fix evaluation of strings containing characters from non-ascii range

If a string contained characters outside the ASCII range, lldb-mi would
print them as hexadecimal codes.  This patch fixes this behaviour by
converting to UTF-8 instead, by having lldb-mi use registered type
summary providers, when they are available.  This patch also fixes
incorrect evaluation of some composite types, like std::string, by
having them use a type registered type summary provider.

Based on patch from evgeny.leviant@gmail.com
Reviewed by: ki.stfu, granata.enrico, clayborg
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D13058

llvm-svn: 249597