[source maps] Ensure all valid source maps are added instead of failing with the first invalid one

Summary:
Several lldb-vscode users have noticed that when a source map rule is invalid (because a folder doesn't exist anymore), the rest of the source maps from their configurations are not applied.
This happens because lldb-vscode executes a single "settings set target.source-map" command with all the source maps and LLDB processes them one by one until one fails.

Instead of doing this, we can process in LLDB all the source map rules and apply the valid ones instead of failing fast.

Reviewers: clayborg, labath, kusmour, aadsm

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D77186