This reapplies four commits for the ClangImporter. They broke the build
on master because the associated commits on swift-clang's
swift-5.1-branch never made it to stable due to automerger woes. This
time I'll push everything through manually.
These are the commits being reapplied:
- a42bc62397 "ClangImporter: Share a module cache between CompilerInstances"
- 9eed34235b "ClangImporter: Adjust to r355777, PCMCache => ModuleCache"
- da8a1c57e8 "ClangImporter: Use the new CacheGeneratedPCH in Clang"
- 2134b17a20 "ClangImporter: Test that PCHs are actually cached in memory"
rdar://problem/48545366