[libspirv] Add include guards where appropriate (#18666)

These are technically required by OpenCL but not observed since
downstream we unconditionally build libclc with all extensions enabled.
Since this won't fly upstream, this commit prepares the ground for a
more correct building of these libraries.

Author: frasercrmck

libclc/libspirv/include/libspirv/image/image.h

@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if defined(__opencl_c_images)
+
 _CLC_OVERLOAD _CLC_DECL float __spirv_ImageRead__Rfloat(image2d_t image,
                                                         int2 coord);
 _CLC_OVERLOAD _CLC_DECL float __spirv_ImageRead__Rfloat(image2d_t image,
@@ -113,3 +115,5 @@ _CLC_OVERLOAD _CLC_DECL void __spirv_ImageWrite(image2d_t image, int4 coord,
 _CLC_OVERLOAD _CLC_DECL void __spirv_ImageWrite(image3d_t image, int4 coord,
                                                 half4 texel);
 #endif
+
+#endif

libclc/libspirv/include/libspirv/spirv_types.h

@@ -50,12 +50,16 @@ typedef struct {
   float real, imag;
 } complex_float;
 
+#ifdef cl_khr_fp64
 typedef struct {
   double real, imag;
 } complex_double;
+#endif
 
+#ifdef cl_khr_fp16
 typedef struct {
   half real, imag;
 } complex_half;
+#endif
 
 #endif // CLC_SPIRV_TYPES