GDB/LLDB plugin: correctly handle string escaping and unescaping

Summary:
- Fix wrong implementation of quoteExpression/unquoteExpression, add unit tests for it

quoteExpression is used in MIVariable when creating new watch variables, original code didn't
escape '\\' in the expression, so the user can't input expression containing '\\'.

unqouteExpression in used in MIBreakpointcontroller to extract path from breakpoint notification
emitted from debugger, because GDB escapes these paths twice, so after regular unescape happens when
parsing the MI response, extra escape is needed.

However, in practice, neither GDB nor LLDB can handle reliably paths with special characters
like '\\' or '\"', so no unit test is added to verify the use of unquoteExpression.

- Improve LLDB data formatter output for non-ascii char, which should be displayed in UI correctly.

LLDB emits unicode escape sequence for non-ascii code. Correct unescape them in the UI.

Test Plan:
Added/Changed unit tests:
- GdbTest::testVariablesWatchesQuotes
- LldbTest::testVariablesWatchesQuotes
- LldbTest::testVariablesNonascii

Reviewers: apol, #kdevelop

Reviewed By: apol, #kdevelop

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2375