Enable SwiftCompilerSources for lldb on Windows

[eeckstein]

Description

No response

Reproduction

Once #73247 lands, set the BOOTSTRAPPING mode to hosttools in the build.ps1 script and remove the workaround in the top-level CMakeLists.txt file.
Currently, the lldb build fails with a missing swiftrt dependency.

Expected behavior

The bootstrapping mode should be set in build.ps1 for the unified llvm-project/swift build.

Environment

reproducible with top-of main on Windows

Additional information

No response

[eeckstein]

Fixed with #73417