Reject non-string types in gdb.Value.lazy_string

Currently, gdb.Value.lazy_string will allow the conversion of any
object to a "lazy string".  However, this was never the intent and is
weird besides.  This patch changes this code to correctly throw an
exception in the non-matching cases.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=20769

gdb/testsuite/gdb.python/py-lazy-string.c

@@ -24,6 +24,7 @@ main ()
   const char array[] = "array";
   pointer typedef_ptr = "typedef pointer";
   const char *null = 0;
+  int not_a_string = 23;
 
   return 0; /* break here */
 }

gdb/testsuite/gdb.python/py-lazy-string.exp

@@ -73,3 +73,8 @@ foreach var_spec { { "ptr" "pointer" "const char \\*" -1 } \
 	#gdb_test "python print ($var.lazy_string(length=0).value())" "\"\"" "empty lazy string value"
     }
 }
+
+gdb_py_test_silent_cmd "python nas = gdb.parse_and_eval('not_a_string')" \
+    "get not_a_string" 1
+gdb_test "python print(nas.lazy_string())" \
+    "Cannot make lazy string from this object"