Additional improvements to internal macros

- Allow usage of `#[llvm_versions(..)]` in more places
    - In some cases this requires use of a new macro,
    `#[llvm_versioned_item]`
- Add `#[llvm_enum(LLVM_ENUM_NAME)]` which automatically derives
conversions to and from an LLVM enum and its Rust equivalent, and allows
use of `#[llvm_versions(..)]` on its variants.
- Used the above to clean up the code for `InstructionOpcode`

Author: bitwalker

internal_macros/Cargo.toml

@@ -7,7 +7,7 @@ build = "build.rs"
 
 [dependencies]
 quote = "0.6"
-syn = { version = "0.15", features = ["full"] }
+syn = { version = "0.15", features = ["full", "fold"] }
 proc-macro2 = "0.4"
 
 [build-dependencies]