Fix error positions of undefined builtins and constants (GH-3030)

Currently Cython generates code like this:

     int __Pyx_InitCachedBuiltins(void) {
         __pyx_builtin_NAME = __Pyx_GetBuiltinName(...);
         if (!__pyx_builtin_NAME) __PYX_ERR(1, 44, __pyx_L1_error)
     }

     int __pyx_pymod_exec_MODULE(PyObject *__pyx_pyinit_module) {
         if (__Pyx_InitCachedBuiltins() < 0) __PYX_ERR(1, 1, __pyx_L1_error)
     }

When InitCachedBuiltins and InitCachedConstants call __PYX_ERR, they
pass the file and line where a builtin is used, but then pymod_exec
overwrites it with 1 and 1, and the error message looks like this:

       File "FILE", line 1, in init MODULE.
         import os
     NameError: name 'NAME' is not defined

After this change Cython generates:

     int __pyx_pymod_exec_MODULE(PyObject *__pyx_pyinit_module) {
         if (__Pyx_InitCachedBuiltins() < 0) goto __pyx_L1_error;
     }

and prints:

       File "FILE", line 44, in init MODULE.
         print(NAME)
     NameError: name 'NAME' is not defined

Cython/Compiler/Code.py

@@ -2357,6 +2357,8 @@ class CCodeWriter(object):
     def error_goto(self, pos):
         lbl = self.funcstate.error_label
         self.funcstate.use_label(lbl)
+        if not pos:
+            return 'goto %s;' % lbl
         return "__PYX_ERR(%s, %s, %s)" % (
             self.lookup_filename(pos[0]),
             pos[1],

Cython/Compiler/ModuleNode.py

@@ -2416,10 +2416,10 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
 
         if Options.cache_builtins:
             code.putln("/*--- Builtin init code ---*/")
-            code.put_error_if_neg(self.pos, "__Pyx_InitCachedBuiltins()")
+            code.put_error_if_neg(None, "__Pyx_InitCachedBuiltins()")
 
         code.putln("/*--- Constants init code ---*/")
-        code.put_error_if_neg(self.pos, "__Pyx_InitCachedConstants()")
+        code.put_error_if_neg(None, "__Pyx_InitCachedConstants()")
 
         code.putln("/*--- Global type/function init code ---*/")
 

tests/run/error_pos.srctree

+PYTHON setup.py build_ext --inplace
+PYTHON test_error_pos.py
+
+######## setup.py ###########
+from distutils.core import setup
+from Cython.Build import cythonize
+
+setup(ext_modules=cythonize("error_pos.pyx"))
+
+######## error_pos.pyx ###########
+from os import *
+
+abcdefg(line)
+
+######## test_error_pos.py ###########
+import subprocess
+import sys
+
+cmd = [sys.executable, '-c', 'import error_pos']
+proc = subprocess.Popen(cmd, stderr=subprocess.PIPE)
+_, err = proc.communicate()
+# The error should contain the line number and the line text where the
+# undefined identifier is used.
+assert b'line 3, in init error_pos' and b'abcdefg(line)' in err, err