add a comment for __clc_schar_t type define

Author: wenju-he

libclc/libspirv/include/libspirv/lp64_types.h

@@ -22,6 +22,8 @@ typedef char __clc_vec4_char_t __attribute__((ext_vector_type(4)));
 typedef char __clc_vec8_char_t __attribute__((ext_vector_type(8)));
 typedef char __clc_vec16_char_t __attribute__((ext_vector_type(16)));
 
+// Typedef some signed char types that SPIR-V requires as the destination of
+// certain conversion builtins.
 typedef signed char __clc_schar_t;
 typedef signed char __clc_vec2_schar_t __attribute__((ext_vector_type(2)));
 typedef signed char __clc_vec3_schar_t __attribute__((ext_vector_type(3)));