[lldb] Fix SourceManager::SourceFileCache insertion

Summary:
Lookup and subsequent insert was done using uninitialized
FileSpec object, which caused the cache to be a no-op.

Bug: llvm.org/PR45310

Depends on D76804.

Reviewers: labath, JDevlieghere

Reviewed By: labath

Subscribers: mgorny, jingham, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D76805