[NFC] Refactor the Preprocessor function that handles Macro definitions and rename Arguments to Parameters in Macro Definitions. 
  - Extracted the reading of the tokens out into a separate function.
  - Replace 'Argument' with 'Parameter' when referring to the identifiers of the macro definition (as opposed to the supplied arguments - MacroArgs - during the macro invocation).

This is in preparation for submitting patches for review to implement __VA_OPT__ which will otherwise just keep lengthening the HandleDefineDirective function and making it less comprehensible.

I will also directly update some extra clang tooling that is broken by the change from Argument to Parameter.

Hopefully the bots will stay appeased.

Thanks!

llvm-svn: 308190
diff --git a/clang/lib/CodeGen/MacroPPCallbacks.cpp b/clang/lib/CodeGen/MacroPPCallbacks.cpp
index 6a31dfe..a6f21d8 100644
--- a/clang/lib/CodeGen/MacroPPCallbacks.cpp
+++ b/clang/lib/CodeGen/MacroPPCallbacks.cpp
@@ -26,8 +26,8 @@
 
   if (MI.isFunctionLike()) {
     Name << '(';
-    if (!MI.arg_empty()) {
-      MacroInfo::arg_iterator AI = MI.arg_begin(), E = MI.arg_end();
+    if (!MI.param_empty()) {
+      MacroInfo::param_iterator AI = MI.param_begin(), E = MI.param_end();
       for (; AI + 1 != E; ++AI) {
         Name << (*AI)->getName();
         Name << ',';