[ClangImporter] Hash content for validation when using clang modules

Set it up as part of configuring the modules build. Should help diminish
the amount of clang modules rebuilds due to mtime invalidation.

rdar://problem/57309954

Author: bcardosolopes

lib/ClangImporter/ClangImporter.cpp

@@ -1071,6 +1071,10 @@ ClangImporter::create(ASTContext &ctx, const ClangImporterOptions &importerOpts,
   // read them later.
   instance.getLangOpts().NeededByPCHOrCompilationUsesPCH = true;
 
+  // Make sure to not trigger extra rebuilds on identical files with mismatching
+  // timestamps.
+  instance.getHeaderSearchOpts().ValidateASTInputFilesContent = true;
+
   if (importerOpts.Mode == ClangImporterOptions::Modes::PrecompiledModule)
     return importer;