Fix build with GCC 7.5

A recent change to 'struct field' caused a build failure with GCC
7.5.0, as reported by Tom de Vries:

/data/vries/gdb/src/gdb/gdbtypes.h:721:51: error:
‘field::m_accessibility’ is too small to hold all values of ‘enum
class accessibility’ [-Werror]
   ENUM_BITFIELD (accessibility) m_accessibility : 2;
                                                   ^

Mark Wielaard pointed out that this was a GCC bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51242

This patch works around the bug by changing several members not to be
bitfields.  It reduces the size of the enum's underlying type,
instead.

I also changed m_bitsize to no longer be a bitfield -- that was done
for packing reasons in ancient times, but with m_accessibility not
being a bitfield, this no longer matters.

I removed fn_field::dummy.  In earlier times it was somewhat normal in
gdb to have these dummy fields to keep track of any available padding.
However, since the advent of "ptype/o", there doesn't seem to be any
need for this.

This patch does not change the size of struct field, fn_field, or
decl_field on 64-bit hosts.

gdb/gdbtypes.h

@@ -534,7 +534,7 @@ union field_location
 };
 
 /* Accessibility of a member.  */
-enum class accessibility : unsigned
+enum class accessibility : unsigned char
 {
   /* It's important that this be 0 so that fields default to
      public.  */
@@ -717,8 +717,6 @@ struct field
 
   unsigned int m_artificial : 1;
 
-  /* Accessibility of the field.  */
-  ENUM_BITFIELD (accessibility) m_accessibility : 2;
   /* Whether the field is 'virtual'.  */
   bool m_virtual : 1;
   /* Whether the field is 'ignored'.  */
@@ -728,13 +726,16 @@ struct field
 
   ENUM_BITFIELD(field_loc_kind) m_loc_kind : 3;
 
+  /* Accessibility of the field.  */
+  enum accessibility m_accessibility;
+
   /* * Size of this field, in bits, or zero if not packed.
      If non-zero in an array type, indicates the element size in
      bits (used only in Ada at the moment).
      For an unpacked field, the field's type's length
      says how many bytes the field occupies.  */
 
-  unsigned int m_bitsize : 28;
+  unsigned int m_bitsize;
 
   /* * In a struct or union type, type of this field.
      - In a function or member type, type of this argument.
@@ -1611,8 +1612,6 @@ struct fn_field
 
   unsigned int is_const:1;
   unsigned int is_volatile:1;
-  /* Accessibility of the field.  */
-  ENUM_BITFIELD (accessibility) accessibility : 2;
   unsigned int is_artificial:1;
 
   /* * A stub method only has some fields valid (but they are enough
@@ -1633,9 +1632,8 @@ struct fn_field
 
   ENUM_BITFIELD (dwarf_defaulted_attribute) defaulted : 2;
 
-  /* * Unused.  */
+  /* Accessibility of the field.  */
-
+  enum accessibility accessibility;
-  unsigned int dummy:6;
 
   /* * Index into that baseclass's virtual function table, minus 2;
      else if static: VOFFSET_STATIC; else: 0.  */
@@ -1658,7 +1656,7 @@ struct decl_field
   struct type *type;
 
   /* Accessibility of the field.  */
-  ENUM_BITFIELD (accessibility) accessibility : 2;
+  enum accessibility accessibility;
 };
 
 /* * C++ language-specific information for TYPE_CODE_STRUCT and