[SubstitutionMap] Introduce hasAnySubstitutableParams().

Introduce an operation to check that a given substitution map is non-empty
(i.e., corresponds to a generic signature) and that the generic signature
has type parameters that aren’t bound to concrete types. This is the
appropriate predicate for SIL-and-later to determine whether there
will be any substitutions.

Author: DougGregor

include/swift/AST/SubstitutionMap.h

@@ -133,7 +133,12 @@ class SubstitutionMap {
   lookupConformance(CanType type, ProtocolDecl *proto) const;
 
   /// Whether the substitution map is empty.
-  bool empty() const { return getGenericSignature() == nullptr; }
+  bool empty() const;
+
+  /// Whether the substitution has any substitutable parameters, i.e.,
+  /// it is non-empty and at least one of the type parameters can be
+  /// substituted (i.e., is not mapped to a concrete type).
+  bool hasAnySubstitutableParams() const;
 
   /// Whether the substitution map is non-empty.
   explicit operator bool() const { return !empty(); }

lib/AST/SubstitutionMap.cpp

@@ -91,6 +91,17 @@ GenericSignature *SubstitutionMap::getGenericSignature() const {
   return storage ? storage->getGenericSignature() : nullptr;
 }
 
+bool SubstitutionMap::empty() const {
+  return getGenericSignature() == nullptr;
+}
+
+bool SubstitutionMap::hasAnySubstitutableParams() const {
+  auto genericSig = getGenericSignature();
+  if (!genericSig) return false;
+
+  return !genericSig->areAllParamsConcrete();
+}
+
 bool SubstitutionMap::hasArchetypes() const {
   for (Type replacementTy : getReplacementTypes()) {
     if (replacementTy && replacementTy->hasArchetype())