parse ellipsis only in the context where it can actually be used (PY-7763)

python/testData/psi/EllipsisAsStatement.py

@@ -0,0 +1,5 @@
+def a():
+  ...     # <- highlighted as "Statement seems to have no effect"
+  pass
+
+a()

python/testData/psi/EllipsisAsStatement.txt

@@ -0,0 +1,33 @@
+PyFile:EllipsisAsStatement.py
+  PyFunction('a')
+    PsiElement(Py:DEF_KEYWORD)('def')
+    PsiWhiteSpace(' ')
+    PsiElement(Py:IDENTIFIER)('a')
+    PyParameterList
+      PsiElement(Py:LPAR)('(')
+      PsiElement(Py:RPAR)(')')
+    PsiElement(Py:COLON)(':')
+    PsiWhiteSpace('\n  ')
+    PyStatementList
+      PsiElement(Py:DOT)('.')
+      PsiErrorElement:Statement expected, found Py:DOT
+        <empty list>
+      PsiElement(Py:DOT)('.')
+      PsiErrorElement:Statement expected, found Py:DOT
+        <empty list>
+      PsiElement(Py:DOT)('.')
+      PsiErrorElement:Statement expected, found Py:DOT
+        <empty list>
+      PsiWhiteSpace('     ')
+      PsiComment(Py:END_OF_LINE_COMMENT)('# <- highlighted as "Statement seems to have no effect"')
+      PsiWhiteSpace('\n  ')
+      PyPassStatement
+        PsiElement(Py:PASS_KEYWORD)('pass')
+  PsiWhiteSpace('\n\n')
+  PyExpressionStatement
+    PyCallExpression: a
+      PyReferenceExpression: a
+        PsiElement(Py:IDENTIFIER)('a')
+      PyArgumentList
+        PsiElement(Py:LPAR)('(')
+        PsiElement(Py:RPAR)(')')