[Modules] Fix module hash computation when module file extensions are involved.

Fix an egregious error in with modules and module file extensions, where only the hash code of the module file extension and sysroots would affect the module cache, leading to module file collisions. The effect of this error was likely masked by the old client of module file extensions (Swift) embedding Clang version information.

Author: DougGregor

lib/Frontend/CompilerInvocation.cpp

@@ -2548,7 +2548,9 @@ std::string CompilerInvocation::getModuleHash() const {
   // Extend the signature with the module file extensions.
   const FrontendOptions &frontendOpts = getFrontendOpts();
   for (const auto &ext : frontendOpts.ModuleFileExtensions) {
-    code = ext->hashExtension(code);
+    code = hash_combine(code, ext->hashExtension(code));
+  }
+
   }
 
   // Darwin-specific hack: if we have a sysroot, use the contents and