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.

tools/sil-func-extractor/SILFunctionExtractor.cpp

@@ -272,8 +272,8 @@ int main(int argc, char **argv) {
   if (CI.getASTContext().hadError())
     return 1;
 
-  auto SILMod = performSILGeneration(CI.getMainModule(), CI.getSILTypes(),
-                                     CI.getSILOptions());
+  auto SILMod = performASTLowering(CI.getMainModule(), CI.getSILTypes(),
+                                   CI.getSILOptions());
 
   // Load the SIL if we have a non-SIB serialized module. SILGen handles SIB for
   // us.