relates to:

• SR-2520 Type alias is lost when applying @escaping-ness
• SR-2406 Print attribute((noescape)) in the generated header

Issue Description:

When I went to implement SR-2406, I discovered that function types imported from Clang (including function pointers, block pointers, and typedefs thereof) are imported with no regard for __attribute__((noescape)). The attribute should be preserved in the imported type's extInfo.

Here's a summary of how the flow seems to happen right now, from what I saw:

• To import a typedef decls, the importer just calls getUnderlyingType() and imports that type:
  

  swift/lib/ClangImporter/ImportDecl.cpp

  Lines 1685 to 1690 in 1a88c86

  	clang::QualType ClangType = Decl->getUnderlyingType();
  	SwiftType = Impl.importType(ClangType,
  	ImportTypeKind::Typedef,
  	isInSystemModule(DC),
  	ClangType->isBlockPointerType(),
  	OTK_Optional);

• To import a block pointer type, it just imports the underlying function pointer:
  

  swift/lib/ClangImporter/ImportType.cpp

  Lines 379 to 382 in 1a88c86

  	Type pointeeType = Impl.importType(type->getPointeeType(),
  	ImportTypeKind::Abstract,
  	AllowNSUIntegerAsInt,
  	CanFullyBridgeTypes);

• Then, here's where parameters of function pointer types are imported:
  

  swift/lib/ClangImporter/ImportType.cpp

  Lines 482 to 496 in 1a88c86

  	for (auto param = type->param_type_begin(),
  	paramEnd = type->param_type_end();
  	param != paramEnd; ++param) {
  	auto swiftParamTy = Impl.importType(*param, ImportTypeKind::Parameter,
  	AllowNSUIntegerAsInt,
  	CanFullyBridgeTypes,
  	OTK_Optional);
  	if (!swiftParamTy)
  	return Type();
  	// FIXME: If we were walking TypeLocs, we could actually get parameter
  	// names. The probably doesn't matter outside of a FuncDecl, which
  	// we'll have to special-case, but it's an interesting bit of data loss.
  	params.push_back(swiftParamTy);
  	}

• At that point, it seems like the noescape information is not available. Other places which use it are checking hasAttr<clang::NoEscapeAttr>() on a param decl, but this only has access to the param types.