Before this patch, the CC lowering pass was applied only when explicitly
requested by the user. This update changes the default behavior to
always apply the CC lowering pass, with an option to disable it using
the `-fno-clangir-call-conv-lowering` flag if necessary.

The primary objective is to make this pass a mandatory step in the
compilation pipeline. This ensures that future contributions correctly
implement the CC lowering for both existing and new targets, resulting
in more consistent and accurate code generation.

From an implementation perspective, several `llvm_unreachable`
statements have been substituted with a new `assert_or_abort` macro.
This macro can be configured to either trigger a non-blocking assertion
or a blocking unreachable statement. This facilitates a test-by-testa
incremental development as it does not required you to know which
code path a test will trigger an just cause a crash if it does.

A few notable changes:

 - Support multi-block function in CC lowering
 - Ignore pointer-related CC lowering
 - Ignore no-proto functions CC lowering
 - Handle missing type evaluation kinds
 - Fix CC lowering for function declarations
 - Unblock indirect function calls
 - Disable CC lowering pass on several tests

…ing through MLIR

…t-cir-flat