ModuleInterface: move -user-module-version to a new field

Titled as "// swift-module-flags-ignorable:", this new field contains new
frontend arguments that can be safely ignored by the older version of the compiler.
For compilers that don't know the field at all, all arguments in it are ignored.

rdar://78233352

lib/Serialization/SerializedModuleLoader.cpp

@@ -22,7 +22,10 @@
 #include "swift/Basic/STLExtras.h"
 #include "swift/Basic/SourceManager.h"
 #include "swift/Basic/Version.h"
+#include "swift/Option/Options.h"
 
+#include "llvm/Option/OptTable.h"
+#include "llvm/Option/ArgList.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/Support/Debug.h"
@@ -970,6 +973,33 @@ bool swift::extractCompilerFlagsFromInterface(StringRef buffer,
     return true;
   assert(FlagMatches.size() == 2);
   llvm::cl::TokenizeGNUCommandLine(FlagMatches[1], ArgSaver, SubArgs);
+  SmallVector<StringRef, 1> IgnFlagMatches;
+  // Cherry-pick supported options from the ignorable list.
+  auto IgnFlagRe = llvm::Regex("^// swift-module-flags-ignorable:(.*)$",
+                               llvm::Regex::Newline);
+  // It's OK the interface doesn't have the ignorable list, we just ignore them
+  // all.
+  if (!IgnFlagRe.match(buffer, &IgnFlagMatches))
+    return false;
+  SmallVector<const char *, 8> IgnSubArgs;
+  llvm::cl::TokenizeGNUCommandLine(IgnFlagMatches[1], ArgSaver, IgnSubArgs);
+  std::unique_ptr<llvm::opt::OptTable> table = swift::createSwiftOptTable();
+  unsigned missingArgIdx = 0;
+  unsigned missingArgCount = 0;
+  auto parsedIgns = table->ParseArgs(IgnSubArgs, missingArgIdx, missingArgCount);
+  for (auto parse: parsedIgns) {
+    // Check if the option is a frontend option. This will filter out unknown
+    // options and input-like options.
+    if (!parse->getOption().hasFlag(options::FrontendOption))
+      continue;
+    // Push the supported option and its value to the list.
+    // We shouldn't need to worry about cases like -tbd-install_name=Foo because
+    // the parsing function should have droped alias options already.
+    SubArgs.push_back(ArgSaver.save(parse->getSpelling()).data());
+    for (auto value: parse->getValues())
+      SubArgs.push_back(value);
+  }
+
   return false;
 }