Updating them to libclang-13-dev was a good change, but we can go even
further:
* The suggestion didn't apply to MacOS.
* The suggestion would need to be updated with every LLVM release,
which is a maintenance burden.
* The suggestion is wrong when compiling with `-tags=llvm12` for
example to choose a different LLVM version.
Therefore, link to the build documentation instead.
This commit also adds a bit of version independence, in particular for
external commands. It also adds the LLVM version to the `tinygo version`
command, which might help while debugging.
Enum types are implemented as named types (with possible accompanying
typedefs as type aliases). The constants inside the enums are treated as
Go constants like in the Go toolchain.
This is a big commit that does a few things:
* It moves CGo processing into a separate package. It never really
belonged in the loader package, and certainly not now that the
loader package may be refactored into a driver package.
* It adds support for multiple CGo files (files that import package
"C") in a single package. Previously, this led to multiple
definition errors in the Go typecheck phase because certain C
symbols were defined multiple times in all the files. Now it
generates a new fake AST that defines these, to avoid multiple
definition errors.
* It improves debug info in a few edge cases that are probably not
relevant outside of bugs in cgo itself.
