[Expressions] Add support of expressions evaluation in some object's context

Summary:
This patch adds support of expression evaluation in a context of some object.
Consider the following example:
```
struct S {
  int a = 11;
  int b = 12;
};

int main() {
  S s;
  int a = 1;
  int b = 2;
  // We have stopped here
  return 0;
}
```
This patch allows to do something like that:
```
lldb.frame.FindVariable("s").EvaluateExpression("a + b")
```
and the result will be `33` (not `3`) because fields `a` and `b` of `s` will be
used (not locals `a` and `b`).

This is achieved by replacing of `this` type and object for the expression. This
has some limitations: an expression can be evaluated only for values located in
the debuggee process memory (they must have an address of `eAddressTypeLoad`
type).

Reviewers: teemperor, clayborg, jingham, zturner, labath, davide, spyffe, serge-sans-paille

Reviewed By: jingham

Subscribers: abidh, lldb-commits, leonid.mashinskiy

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D55318

llvm-svn: 353149