Use diagnostic name "macro expansion" without the "expression" for macro expansion expressions

These show up in declaration positions now, and we can only determine
what kind of entity it is once we know more about the macro
(semantically).

Author: DougGregor

gyb_syntax_support/ExprNodes.py

@@ -648,7 +648,7 @@
 
     # e.g., "#embed("filename.txt")"
     Node('MacroExpansionExpr',
-         name_for_diagnostics="macro expansion expression", kind='Expr',
+         name_for_diagnostics="macro expansion", kind='Expr',
          traits=['FreestandingMacroExpansion'],
          children=[
              Child('PoundToken', kind='PoundToken',