Support debug information for 64-bit or unsigned enums

[HertzDevil]

Previously, Crystal assumed all enums are 32-bit and signed when emitting debug information:

enum SignedEnum : Int64
  X = 0x0123_4567_89ab_cdef_i64
end

enum UnsignedEnum : UInt64
  Y = 0xfedc_ba98_7654_3210_u64
end

x = SignedEnum::X
y = UnsignedEnum::Y
debugger

(lldb) p x
(SignedEnum) 81985529216486895
(lldb) p y
(UnsignedEnum) 18364758544493064720
(lldb) p (int64_t)SignedEnum::X
(int64_t) -1985229329
(lldb) p (uint64_t)UnsignedEnum::Y
(uint64_t) 1985229328

Support for 64-bit or unsigned values is present in all LLVM versions we support. This PR leverages that:

(lldb) p x
(SignedEnum) X
(lldb) p y
(UnsignedEnum) Y
(lldb) p (int64_t)SignedEnum::X
(int64_t) 81985529216486895
(lldb) p (uint64_t)UnsignedEnum::Y
(uint64_t) 18364758544493064720

Enum values larger than 64 bits require LLVM 11, plus a corresponding C API.