NFC: Rename performSILGeneration -> performASTLowering

And also rename the underlying request and
descriptor.

This rename is motivated by the fact that the
operation may instead perform parsing of SIL files
and/or deserialization of SIB files.

lib/Frontend/ModuleInterfaceBuilder.cpp

@@ -209,7 +209,7 @@ bool ModuleInterfaceBuilder::buildSwiftModuleInternal(
     SILOptions &SILOpts = subInvocation.getSILOptions();
     auto Mod = SubInstance.getMainModule();
     auto &TC = SubInstance.getSILTypes();
-    auto SILMod = performSILGeneration(Mod, TC, SILOpts);
+    auto SILMod = performASTLowering(Mod, TC, SILOpts);
     if (!SILMod) {
       LLVM_DEBUG(llvm::dbgs() << "SILGen did not produce a module\n");
       return true;