clc: Rework logging a bit

clc: Rework logging a bit

First, separate out the LLVM context logging to make it take a
clc_logger instead of passing in a string stream.  Currently, the LLVM
context may outlive the string stream which we assign which may lead to
use-after-free errors.  Second, use a separate string stream for clang
diagnosticl logging which we intentionally declare before the compiler
so the compiler can't outlive it.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Icecream95 <ixn@disroot.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15937>