Fix formatting of compound-type variable declarations in macros (#6997)

By default clang-format interprets function-like macro invocations as
function calls. E.g. the argument of `CARBON_KIND(llvm::ListSeparator*
sep)` is interpreted as an expression, meaning the `*` is an infix
binary operator, so it inserts a space before the `*`. This change
teaches clang-format that `CARBON_KIND(x)` and
`CARBON_ASSIGN_OR_RETURN(x)` rewrite to `x`, which is close enough to
the truth to enable it to format them correctly. See the [clang-format
docs](https://clang.llvm.org/docs/ClangFormatStyleOptions.html#macros)
for details.

toolchain/sem_ir/stringify.cpp

@@ -176,7 +176,7 @@ class StepStack {
           PushSpecificNamedConstraint(specific_named_constraint);
           break;
         }
-        case CARBON_KIND(llvm::ListSeparator * sep): {
+        case CARBON_KIND(llvm::ListSeparator* sep): {
           PushString(*sep);
           break;
         }