[lldb] Add expect_expr function for testing expression evaluation in dotests.

Summary:
This patch adds a new function to lldbtest: `expect_expr`. This function is supposed to replace the current approach
of calling `expect`/`runCmd` with `expr`, `p` etc.

`expect_expr` allows evaluating expressions and matching their value/summary/type/error message without
having to do any string matching that might allow unintended passes (e.g., `self.expect("expr 3+4", substrs=["7"])`
can unexpectedly pass for results like `(Class7) $0 = 7`, `(int) $7 = 22`, `(int) $0 = 77` and so on).

This only uses the function in a few places to test and demonstrate it. I'll migrate the tests in follow up commits.

Reviewers: JDevlieghere, shafik, labath

Reviewed By: labath

Subscribers: christof, abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D70314

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