ClangImporter: Use the new CacheGeneratedPCH in Clang

This flag causes the generated PCH to be written into the in-memory
module cache.  Every CompilerInstance that tries to read the PCH will
use it rather than reading a potentially-since-overwritten version from
the filesystem.

rdar://problem/48740787
(cherry picked from commit 236fc19121)

lib/ClangImporter/ClangImporter.cpp

@@ -1440,6 +1440,7 @@ ClangImporter::emitBridgingPCH(StringRef headerPath,
   invocation->getFrontendOpts().ProgramAction = clang::frontend::GeneratePCH;
   invocation->getPreprocessorOpts().resetNonModularOptions();
   invocation->getLangOpts()->NeededByPCHOrCompilationUsesPCH = true;
+  invocation->getLangOpts()->CacheGeneratedPCH = true;
 
   clang::CompilerInstance emitInstance(
     Impl.Instance->getPCHContainerOperations(),