Add static_asserts for the sizes of space-critical structs

Specifically the three structs mentioned in symtab.h:
- general_symbol_info
- symbol
- partial_symbol

This ensures that those structs won't accidentally get bigger.

gdb/ChangeLog:

2019-11-04  Christian Biesinger  <cbiesinger@google.com>

	* psympriv.h: Add static_asserts for sizeof (general_symbol_info)
	and sizeof (symbol).
	* symtab.h: Add a static_assert for sizeof (partial_symbol).

Change-Id: Idd68320aa3e79ee7cc749019724636a58ce4b9c6

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2019-11-04  Christian Biesinger  <cbiesinger@google.com>
+
+	* psympriv.h: Add static_asserts for sizeof (general_symbol_info)
+	and sizeof (symbol).
+	* symtab.h: Add a static_assert for sizeof (partial_symbol).
+
 2019-11-04  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
 
 	* NEWS (Changes since GDB 8.3): Document Solaris 10 removal.

gdb/psympriv.h

@@ -82,6 +82,12 @@ struct partial_symbol
   ENUM_BITFIELD(address_class) aclass : SYMBOL_ACLASS_BITS;
 };
 
+/* This struct is size-critical (see comment at the to of symtab.h), so this
+   assert makes sure the size doesn't change accidentally.  Be careful when
+   purposely increasing the size.  */
+gdb_static_assert ((sizeof (void *) == 8 && sizeof (partial_symbol) == 40)
+		   || (sizeof (void *) == 4 && sizeof (partial_symbol) == 24));
+
 /* A convenience enum to give names to some constants used when
    searching psymtabs.  This is internal to psymtab and should not be
    used elsewhere.  */

gdb/symtab.h

@@ -446,6 +446,13 @@ struct general_symbol_info
   short section;
 };
 
+/* This struct is size-critical (see comment at the top), so this assert
+   makes sure the size doesn't change accidentally.  Be careful when
+   purposely increasing the size.  */
+gdb_static_assert ((sizeof (void *) == 8 && sizeof (general_symbol_info) == 32)
+		   || (sizeof (void *) == 4 &&
+		       sizeof (general_symbol_info) == 20));
+
 extern void symbol_set_demangled_name (struct general_symbol_info *,
 				       const char *,
                                        struct obstack *);
@@ -1187,6 +1194,12 @@ struct symbol
   struct symbol *hash_next;
 };
 
+/* This struct is size-critical (see comment at the top), so this assert
+   makes sure the size doesn't change accidentally.  Be careful when
+   purposely increasing the size.  */
+gdb_static_assert ((sizeof (void *) == 8 && sizeof (symbol) == 72)
+		   || (sizeof (void *) == 4 && sizeof (symbol) == 40));
+
 /* Several lookup functions return both a symbol and the block in which the
    symbol is found.  This structure is used in these cases.  */