[NFC] Change forEachFunctionParam to only ignore the final orig parameter

and not also drop a subst parameter.

This turned out to be more convenient for certain clients (e.g. SILGenPoly)
than requiring the full subst param list to be passed in.  These clients
want to process the subst param list separately, and dropping self early
can be convenient for that.  The only fundamental reason we need this
flag is for working with the orig type, so just use it for that; clients
that need to use this feature can reasonably be expected to cooperate.

include/swift/SIL/AbstractionPatternGenerators.h

@@ -81,7 +81,7 @@ class FunctionParamGenerator {
 public:
   FunctionParamGenerator(AbstractionPattern origFunctionType,
                          AnyFunctionType::CanParamArrayRef substParams,
-                         bool ignoreFinalParam);
+                         bool ignoreFinalOrigParam);
 
   /// Is the traversal finished?  If so, none of the getters below
   /// are allowed to be called.

lib/SIL/IR/AbstractionPattern.cpp

@@ -1203,7 +1203,7 @@ unsigned AbstractionPattern::getNumFunctionParams() const {
 
 void AbstractionPattern::
 forEachFunctionParam(AnyFunctionType::CanParamArrayRef substParams,
-                     bool ignoreFinalParam,
+                     bool ignoreFinalOrigParam,
          llvm::function_ref<void(unsigned origParamIndex,
                                  unsigned substParamIndex,
                                  ParameterTypeFlags origFlags,
@@ -1216,7 +1216,7 @@ forEachFunctionParam(AnyFunctionType::CanParamArrayRef substParams,
                                  AbstractionPattern origExpansionType,
                         AnyFunctionType::CanParamArrayRef substParams)>
              handleExpansion) const {
-  FunctionParamGenerator generator(*this, substParams, ignoreFinalParam);
+  FunctionParamGenerator generator(*this, substParams, ignoreFinalOrigParam);
 
   for (; !generator.isFinished(); generator.advance()) {
     if (generator.isPackExpansion()) {
@@ -1239,7 +1239,7 @@ forEachFunctionParam(AnyFunctionType::CanParamArrayRef substParams,
 FunctionParamGenerator::FunctionParamGenerator(
                               AbstractionPattern origFunctionType,
                               AnyFunctionType::CanParamArrayRef substParams,
-                              bool ignoreFinalParam)
+                              bool ignoreFinalOrigParam)
     : origFunctionType(origFunctionType), allSubstParams(substParams) {
   origFunctionTypeIsOpaque =
     (origFunctionType.isTypeParameterOrOpaqueArchetype() ||
@@ -1249,11 +1249,8 @@ FunctionParamGenerator::FunctionParamGenerator(
     numOrigParams = allSubstParams.size();
   } else {
     numOrigParams = origFunctionType.getNumFunctionParams();
-  }
+    if (ignoreFinalOrigParam)
-
+      numOrigParams--;
-  if (ignoreFinalParam) {
-    allSubstParams = allSubstParams.drop_back();
-    numOrigParams--;
   }
 
   if (!isFinished()) loadParameter();

lib/SIL/IR/SILFunctionType.cpp

@@ -1569,7 +1569,8 @@ private:
     maybeAddForeignParameters();
 
     // Process all the non-self parameters.
-    origType.forEachFunctionParam(params, hasSelf,
+    origType.forEachFunctionParam(params.drop_back(hasSelf ? 1 : 0),
+                                  /*ignore final orig param*/ hasSelf,
         [&](unsigned origParamIndex, unsigned substParamIndex,
             ParameterTypeFlags origFlags,
             AbstractionPattern origParamType,