[lldb] Add new LLDB setting: use-source-cache

Summary:
LLDB memory-maps large source files, and at the same time, caches
all source files in the Source Cache.

On Windows, memory-mapped source files are not writeable, causing
bad user experience in IDEs (such as errors when saving edited files).
IDEs should have the ability to disable the Source Cache at LLDB
startup, so that users can edit source files while debugging.

Bug: llvm.org/PR45310

Reviewers: labath, JDevlieghere, jingham

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D76804