We were mixing uses of ::llvm:: and llvm::
Since it is a well-known namespace, there's virtually no risk of
unintentionally reusing the same name.
This follows the recommendation of go/totw/151.

Bug: consistency cleanup
Change-Id: I914f7a2d7b76f21f1f0758506319f2dd47cc1832
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/49828Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>