Option to disable printing package-name in public or private interface.

Having package-name flag in non-package interfaces causes them to be built as if
belonging to a package, which causes an issue for a loading client outside of the
package as follows.

For example, when building X that depends on A with the following dependency chain:
  X --> A --> B --(package-only)--> C

1. X itself is not in the same package as A, B, and C.
2. When dependency scanning X, and opening up B, because the scan target is in a
   different package domain, the scanner decides that B's package-only dependency
   on C is to be ignored.
3. When then finally building A itself, it will load its dependencies, but because
   the .private.swiftinterface of A still specifies -package-name, when it loads
   B, it will then examine its dependencies and deem that this package-only dependency
   on C is required.

Because (2) and (3) disagree, we get an error now when building the private A textual interface.

rdar://130701866

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -60,8 +60,35 @@ static void printToolVersionAndFlagsComment(raw_ostream &out,
       << InterfaceFormatVersion << "\n";
   out << "// " SWIFT_COMPILER_VERSION_KEY ": "
       << ToolsVersion << "\n";
+
+  // Check if printing package-name is disabled for
+  // non-package interfaces (by default, it's printed
+  // in all interfaces).
+  std::string flagsStr = Opts.Flags;
+  if (Opts.DisablePackageNameForNonPackageInterface &&
+      !Opts.printPackageInterface()) {
+    size_t pkgIdx = 0;
+    size_t end = flagsStr.size();
+    auto pkgFlag = StringRef("-package-name ");
+    size_t pkgLen = pkgFlag.size();
+
+    // Find the package-name flag and its value and
+    // drop them. There can be multiple package-name
+    // flags passed, so drop them all.
+    while (pkgIdx < end) {
+      // First, find "-package-name "
+      pkgIdx = flagsStr.find(pkgFlag, 0);
+      if (pkgIdx == std::string::npos)
+        break;
+      // If found, find the next flag's starting pos.
+      auto next = flagsStr.find_first_of("-", pkgIdx + pkgLen + 1);
+      // Remove the substr in-place.
+      flagsStr.erase(pkgIdx, next - pkgIdx);
+    }
+  }
+
   out << "// " SWIFT_MODULE_FLAGS_KEY ": "
-      << Opts.Flags;
+      << flagsStr;
 
   // Insert additional -module-alias flags
   if (Opts.AliasModuleNames) {