clang/Modules: Sink CompilerInstance::KnownModules into ModuleMap
Avoid use-after-frees when FrontendAction::BeginSourceFile is called
twice on the same CompilerInstance by sinking
CompilerInstance::KnownModules into ModuleMap. On the way, rename the
map to CachedModuleLoads. I considered (but rejected) merging this with
ModuleMap::Modules, since that only has top-level modules and this map
This is an alternative to https://reviews.llvm.org/D58497. Thanks to
nemanjai for the detailed analysis of the problem!