[naga wgsl-in] Move builtin function calls into their own fn.

Move the code for handling calls to WGSL builtin functions out of
`Lowerer::call` and into its own new function,
`Lowerer::call_builtin`, for clarity.

Consolidate `bitcast` and `coopLoad` handling with the other
functions. Do template parameter list finalization in the caller.

Author: jimblandy