[ClangImporter] Retire the term "adapter" in favor of "overlay" (#24427)

Way back in Swift 1 I was trying to draw a distinction between
"overlays", separate libraries that added Swift content to an existing
Objective-C framework, and "the Swift part of a mixed-source
framework", even though they're implemented in almost exactly the same
way. "Adapter module" was the term that covered both of those. In
practice, however, no one knew what "adapter" meant. Bring an end to
this confusion by just using "overlay" within the compiler even for
the mixed-source framework case.

No intended functionality change.

lib/Frontend/CompilerInvocation.cpp

@@ -558,7 +558,7 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts,
 
   Opts.DisableModulesValidateSystemHeaders |= Args.hasArg(OPT_disable_modules_validate_system_headers);
 
-  Opts.DisableAdapterModules |= Args.hasArg(OPT_emit_imported_modules);
+  Opts.DisableOverlayModules |= Args.hasArg(OPT_emit_imported_modules);
 
   if (const Arg *A = Args.getLastArg(OPT_pch_output_dir)) {
     Opts.PrecompiledHeaderOutputDir = A->getValue();