[lldb] Make Fix-Its also apply to top-level expressions

Summary:
Currently top-level expressions won't automatically get Fix-Its applied. The reason
for that is that we only set the `m_fixed_text` member if we have a wrapping
source code (I.e. `m_source_code` is not zero and is wrapping some expressions).

This patch just always sets `m_fixed_text` to get this working.

Reviewers: labath, jingham

Reviewed By: labath

Subscribers: JDevlieghere

Differential Revision: https://reviews.llvm.org/D77042