AST: Don't canonicalize GenericFunctionTypes in TypeBase::getASTContext()

A TypeBase stores either an ASTContext if its canonical, or its canonical
type if it is not.

getASTContext() will then canonicalize the type if its not canonical.
While this is very clever, in the case of a GenericFunctionType, it
triggers the rather expensive creation of a GenericSignatureBuilder.

Avoid this by special-casing GenericFunctionType here and getting the
ASTContext from a structural sub-component that is known to exist
instead.

Author: slavapestov

include/swift/AST/Types.h

@@ -492,13 +492,7 @@ class alignas(1 << TypeAlignInBits) TypeBase {
   TypeBase *getWithoutSyntaxSugar();
 
   /// getASTContext - Return the ASTContext that this type belongs to.
-  ASTContext &getASTContext() {
-    // If this type is canonical, it has the ASTContext in it.
-    if (isCanonical())
-      return *const_cast<ASTContext*>(Context);
-    // If not, canonicalize it to get the Context.
-    return *const_cast<ASTContext*>(getCanonicalType()->Context);
-  }
+  ASTContext &getASTContext();
 
   /// isEqual - Return true if these two types are equal, ignoring sugar.
   ///
@@ -5994,6 +5988,22 @@ class PlaceholderType : public TypeBase {
 };
 DEFINE_EMPTY_CAN_TYPE_WRAPPER(PlaceholderType, Type)
 
+/// getASTContext - Return the ASTContext that this type belongs to.
+inline ASTContext &TypeBase::getASTContext() {
+  // If this type is canonical, it has the ASTContext in it.
+  if (isCanonical())
+    return *const_cast<ASTContext*>(Context);
+
+  // getCanonicalType() on GenericFunctionType is very expensive;
+  // instead we can more easily fish the ASTContext out of any
+  // structural sub-component.
+  if (auto *genericFnType = dyn_cast<GenericFunctionType>(this))
+    return genericFnType->getGenericParams()[0]->getASTContext();
+
+  // If not, canonicalize it to get the Context.
+  return *const_cast<ASTContext*>(getCanonicalType()->Context);
+}
+
 inline bool TypeBase::isTypeVariableOrMember() {
   if (is<TypeVariableType>())
     return true;