[NFC][libspirv] delete clc_core.h which is now irrelevant (#18092)

The file was intended to share code between SPIR-V and OpenCL. Now
shared code are moving to clc.
Also delete unused files: clc_add_sat.cl, clc_mad_sat.cl, clc_sub_sat.cl
and clc_ldexp.cl.
llvm-diff shows no change to libspirv-nvptx64--nvidiacl.bc and
libspirv-amdgcn--amdhsa.bc.

Author: wenju-he

libclc/generic/include/core/clc_core.h

@@ -27,9 +27,6 @@ common/radians.cl
 common/sign.cl
 common/smoothstep.cl
 common/step.cl
-core/integer/clc_add_sat.cl
-core/integer/clc_mad_sat.cl
-core/integer/clc_sub_sat.cl
 geometric/cross.cl
 geometric/distance.cl
 geometric/dot.cl

libclc/libspirv/lib/generic/SOURCES

@@ -66,7 +66,17 @@
    ===----------------------------------------------------------------------===
 */
 
-#include <core/clc_core.h>
+#ifdef cl_khr_fp64
+#pragma OPENCL EXTENSION cl_khr_fp64 : enable
+#endif
+
+#ifdef cl_khr_fp16
+#pragma OPENCL EXTENSION cl_khr_fp16 : enable
+#endif
+
+#include <clc/clcfunc.h>
+#include <clc/clctypes.h>
+#include <core/convert.h>
 #include <libspirv/spirv.h>
 
 #ifdef cl_khr_fp16

libclc/libspirv/lib/generic/core/integer/clc_add_sat.cl

@@ -84,7 +84,6 @@
    THE SOFTWARE.
 */
 
-#include <core/clc_core.h>
 #include <clc/clc_as_type.h>
 #include <clc/integer/clc_abs.h>
 #include <clc/shared/clc_clamp.h>

libclc/libspirv/lib/generic/core/integer/clc_mad_sat.cl

@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include <core/clc_core.h>
+#include <core/integer/clc_mad_sat.h>
 #include <libspirv/spirv.h>
 
 #define __CLC_BODY <mad_sat.inc>

libclc/libspirv/lib/generic/core/integer/clc_sub_sat.cl

@@ -7,9 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 #include <clc/clcmacro.h>
-#include <core/clc_core.h>
-#include <libspirv/spirv.h>
 #include <libspirv/ptx-nvidiacl/libdevice.h>
+#include <libspirv/spirv.h>
 
 _CLC_OVERLOAD _CLC_DEF int __spirv_ocl_s_mul24(int x, int y) {
   return __nv_mul24(x, y);