[gyb] Fix line-directive mapping of multiline substitutions

fixes https://bugs.swift.org/browse/SR-4362

Author: Dave Abrahams

utils/gyb.py

@@ -579,10 +579,8 @@ def append_text(self, text, file, line):
                 elif '\n' in text:
                     i = text.find('\n')
                     self.result_text.append(text[:i + 1])
-                    self.last_file_line = (
-                        self.last_file_line[0], self.last_file_line[1] + 1)
                     # and try again
-                    self.append_text(text[i + 1:], file, line)
+                    self.append_text(text[i + 1:], file, line + 1)
                     return
 
         self.result_text.append(text)
@@ -755,10 +753,16 @@ def expand(filename, line_directive=_default_line_directive, **local_bindings):
     >>> from tempfile import NamedTemporaryFile
     >>> f = NamedTemporaryFile()
     >>> f.write(
-    ... '''---
+    ... r'''---
     ... % for i in range(int(x)):
     ... a pox on ${i} for epoxy
     ... % end
+    ... ${120 +
+    ... 
+    ...    3}
+    ... abc
+    ... ${"w\nx\nX\ny"}
+    ... z
     ... ''')
     >>> f.flush()
     >>> result = expand(
@@ -772,7 +776,16 @@ def expand(filename, line_directive=_default_line_directive, **local_bindings):
     a pox on 0 for epoxy
     //#sourceLocation(file: "dummy.file", line: 3)
     a pox on 1 for epoxy
-
+    //#sourceLocation(file: "dummy.file", line: 5)
+    123
+    //#sourceLocation(file: "dummy.file", line: 8)
+    abc
+    w
+    x
+    X
+    y
+    //#sourceLocation(file: "dummy.file", line: 10)
+    z
     """
     with open(filename) as f:
         t = parse_template(filename, f.read())