Make ptype/whatis print function name of functions with no debug info too

The patch to make GDB stop assuming functions return int left GDB with
an inconsistency.  While with normal expression evaluation the
"unknown return type" error shows the name of the function that misses
debug info:

  (gdb) p getenv ("PATH")
  'getenv' has unknown return type; cast the call to its declared return type
   ^^^^^^

which is handy in more complicated expressions, "ptype" does not:

  (gdb) ptype getenv ("PATH")
  function has unknown return type; cast the call to its declared return type
  ^^^^^^^^

This commit builds on the new OP_VAR_MSYM_VALUE to fix it, by making
OP_FUNCALL extract the function name from the symbol stored in
OP_VAR_VALUE/OP_VAR_MSYM_VALUE.  We now get the same error in "print"
vs "ptype":

  (gdb) ptype getenv()
  'getenv' has unknown return type; cast the call to its declared return type
  (gdb) p getenv()
  'getenv' has unknown return type; cast the call to its declared return type

gdb/ChangeLog:
2017-09-04  Pedro Alves  <palves@redhat.com>

	* eval.c (evaluate_subexp_standard): <OP_FUNCALL>: Extract
	function name from symbol/minsym and pass it to
	error_call_unknown_return_type.

gdb/testsuite/ChangeLog:
2017-09-04  Pedro Alves  <palves@redhat.com>

	* gdb.base/nodebug.exp: Test that ptype's error about functions
	with unknown return type includes the function name too.

1 files changed, 13 insertions, 1 deletions

diff --git a/gdb/eval.c b/gdb/eval.c
index 457e280..0e77f0a 100644
--- a/gdb/eval.c
+++ b/gdb/eval.c
@@ -716,6 +716,7 @@ evaluate_subexp_standard (struct type *expect_type,
   int save_pos1;
   struct symbol *function = NULL;
   char *function_name = NULL;
+  const char *var_func_name = NULL;
 
   pc = (*pos)++;
   op = exp->elts[pc].opcode;
@@ -1545,6 +1546,17 @@ evaluate_subexp_standard (struct type *expect_type,
 	    }
 	  else
 	    {
+	      if (op == OP_VAR_MSYM_VALUE)
+		{
+		  symbol *sym = exp->elts[*pos + 2].symbol;
+		  var_func_name = SYMBOL_PRINT_NAME (sym);
+		}
+	      else if (op == OP_VAR_VALUE)
+		{
+		  minimal_symbol *msym = exp->elts[*pos + 2].msymbol;
+		  var_func_name = MSYMBOL_PRINT_NAME (msym);
+		}
+
 	      argvec[0] = evaluate_subexp_with_coercion (exp, pos, noside);
 	      type = value_type (argvec[0]);
 	      if (type && TYPE_CODE (type) == TYPE_CODE_PTR)
@@ -1761,7 +1773,7 @@ evaluate_subexp_standard (struct type *expect_type,
 		return_type = expect_type;
 
 	      if (return_type == NULL)
-		error_call_unknown_return_type (NULL);
+		error_call_unknown_return_type (var_func_name);
 
 	      return allocate_value (return_type);
 	    }