Rename splits for intN_t tests (#5615)

Renaming the splits in the tests for `int16_t` and `int32_t` to
emphasize testing typedefs.

Part of #5263 .

toolchain/check/testdata/interop/cpp/function_param_int16.carbon

@@ -152,7 +152,7 @@ fn F() {
 }
 
 // ============================================================================
-// int16_t
+// typedef for short
 // ============================================================================
 
 // --- int16_t.h

toolchain/check/testdata/interop/cpp/function_param_int32.carbon

@@ -116,7 +116,7 @@ fn F() {
 }
 
 // ============================================================================
-// int32_t
+// typedef for int
 // ============================================================================
 
 // --- int32_t.h