It's only used for mangling the names of our generated functions during
Routine construction, so don't hang on to it for the entire lifetime of
the routine.

Also, don't set Function attributes during routine compilation, set them
at creation.

Bug: b/177024837
Change-Id: I974b10430bced5d59f51e54426f7ee0aa8c96ca2
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51569
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>