Merge remote-tracking branch 'origin/swift-4.0-branch' into stable
* origin/swift-4.0-branch:
[Modules] Fallback to building the module if a timeout occurs
Modules: Use a PCMCache to manage memory buffers
Modules: Hash the PCM content and use it as SIGNATURE
Modules: Simplify CompilerInstance constructor, NFC
Modules: Fix a minor performance bug from r293393
Modules: Simplify the ModuleFile constructor; likely NFC
Modules: Return early in ModuleManager::addModule; NFC
Modules: Clean up ModuleFile::Imports in ModuleManager::removeModules
Modules: Enforce that ModuleManager::removeModules deletes the tail
Modules: Clarify ownership of ModuleFile instances in ModuleManager, NFC
Modules: Return ModuleFile& from ModuleManager::begin, etc.; NFC
Modules: Separate out a checkSignature helper, almost NFC
