[lldb] Replace StringConvert with llvm::to_integer when parsing integer values in CommandObjects

Summary:

This replaces the current use of LLDB's own `StringConvert` with LLVM's
`to_integer` which has a less error-prone API and doesn't use special 'error
values' to designate parsing problems.

Where needed I also added missing error handling code that prints a parsing
error instead of continuing with the error value returned from `StringConvert`
(which either gave a cryptic error message or just took the error value
performed an incorrect action with it. For example, `frame recognizer delete -1`
just deleted the frame recognizer at index 0).

Reviewers: #lldb, labath

Reviewed By: labath

Subscribers: labath, abidh, JDevlieghere

Differential Revision: https://reviews.llvm.org/D82297