[lldb] Use one Progress event per root module build

[lldb] Use one Progress event per root module build

Following the work done by @jdevlieghere in D143690, this changes how Clang module build
events are emitted.

Instead of one Progress event per module being built, a single Progress event is used to
encompass all modules, and each module build is sent as an `Increment` update.

Differential Revision: https://reviews.llvm.org/D147248