llvm: Update version from 4.0.1 to 6.0.0

Rust 1.25.0 requires clang 6.0.0.

[Release notes for LLVM 6.0.0][1] (excerpt):

> ### Non-comprehensive list of changes in this release ###
>
> *   Support for retpolines was added to help mitigate “branch target
> injection” (variant #2) of the “Spectre” speculative side channels
> described by Project Zero and the Spectre paper.
> *   The Redirects argument of llvm::sys::ExecuteAndWait and
> llvm::sys::ExecuteNoWait was changed to an ArrayRef of optional
> StringRef‘s to make it safer and more convenient to use.
> *   The backend name was added to the Target Registry to allow
> run-time information to be fed back into TableGen. Out-of-tree targets
> will need to add the name used in the def X : Target definition to the
> call to RegisterTarget.
> *   The Debugify pass was added to opt to facilitate testing of debug
> info preservation. This pass attaches synthetic DILocations and
> DIVariables to the instructions in a Module. The CheckDebugify pass
> determines how much of the metadata is lost.
> *   Significantly improved quality of CodeView debug info for Windows.
> *   Preliminary support for Sanitizers and sibling features on
> X86(_64) NetBSD (ASan, UBsan, TSan, MSan, SafeStack, libFuzzer).

[1]: https://releases.llvm.org/6.0.0/docs/ReleaseNotes.html

llvm.be0

@@ -1,6 +1,6 @@
 #!/usr/bin/env beesh
 
-# BEE_VERSION llvm-4.0.1-0
+# BEE_VERSION llvm-6.0.1-0
 
 ## this file was created by bee init and should be executed to build a
 ## bee-package. (Additional hints are located at the end of this file.)