gdb: Make printing enum types prettier.

gdb: Make printing enum types prettier.

Update printing of enum types to follow the same formatting
conventions as for structs resulting in more readable output.
Now that horizontal space is less of an issue enum values are
always printed.
Empty enums are now printed with an added "<no enum values>"
message.

Example of printing an enum with default values:
enum class TestEnum {A, B, C, D};

(gdb) ptype TestEnum
type = enum class TestEnum : int {
    TestEnum::A = 0,
    TestEnum::B = 1,
    TestEnum::C = 2,
    TestEnum::D = 3
}

Example of printing an empty enum:
enum class TestEnum {};

(gdb) ptype TestEnum
type = enum class TestEnum : int {
    <no enum values>
}

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=19294
Approved-By: Tom Tromey <tom@tromey.com>