Re-index project when updating the toolchain

[ahoppen]

When switching to a different toolchain, the index-build folder isn’t really in a valid state anymore (eg. all the preparation state is out-of-date). We should re-index the project when we detect that SourceKit-LSP is launched with a different toolchain than the one used to index the project.

[ahoppen]

Synced to Apple’s issue tracker as rdar://141087885