[lldb][ClangUserExpression][NFCI] Pass the most specific ExecutionContextScope...

[lldb][ClangUserExpression][NFCI] Pass the most specific ExecutionContextScope possible into ClangExpressionParser (#87657)

The `ClangExpressionParser` takes an `ExecutionContextScope` which it
uses to query the `Process`/`Target`/`StackFrame` to set various
compiler options in preparation for parsing an expression.

However, `TryParse` constructs the parser with a `Process` or `Target`,
never a `StackFrame`. So when the parser tries to retrieve the current
`StackFrame` from the `exe_scope`, it doesn't succeed. In future patches
we want to query the `StackFrame` from within the
`ClangExpressionParser` constructor.

This patch simplifies `TryParse`, by removing the redundant `exe_scope`
parameter, and instead uses the `exe_ctx` to derive the most fitting
`exe_scope` to pass into `ClangExpressionParser`.

Not entirely sure how to test this. This patch is a prerequisite to get
subsequent patches that set `LangOpts` based on the current `StackFrame`
to work.