[OpenCL] Add const attribute to ctz() builtin

[svenvh]

LGTM, thanks!

clang/lib/Headers/opencl-c.h

 ulong16 __ovld __cnfn clz(ulong16 x);
 
 /**
  * Returns the count of trailing 0-bits in x. If x is 0,
  * returns the size in bits of the type of x or
  * component type of x, if x is a vector.
  */
 #if defined(__OPENCL_CPP_VERSION__) || (__OPENCL_C_VERSION__ >= CL_VERSION_2_0)
-char __ovld ctz(char x);
-uchar __ovld ctz(uchar x);
-char2 __ovld ctz(char2 x);
-uchar2 __ovld ctz(uchar2 x);
-char3 __ovld ctz(char3 x);
-uchar3 __ovld ctz(uchar3 x);
-char4 __ovld ctz(char4 x);
-uchar4 __ovld ctz(uchar4 x);
-char8 __ovld ctz(char8 x);
-uchar8 __ovld ctz(uchar8 x);
-char16 __ovld ctz(char16 x);
-uchar16 __ovld ctz(uchar16 x);
-short __ovld ctz(short x);
-ushort __ovld ctz(ushort x);
-short2 __ovld ctz(short2 x);
-ushort2 __ovld ctz(ushort2 x);
-short3 __ovld ctz(short3 x);
-ushort3 __ovld ctz(ushort3 x);
-short4 __ovld ctz(short4 x);
-ushort4 __ovld ctz(ushort4 x);
-short8 __ovld ctz(short8 x);
-ushort8 __ovld ctz(ushort8 x);
-short16 __ovld ctz(short16 x);
-ushort16 __ovld ctz(ushort16 x);
-int __ovld ctz(int x);
-uint __ovld ctz(uint x);
-int2 __ovld ctz(int2 x);
-uint2 __ovld ctz(uint2 x);
-int3 __ovld ctz(int3 x);
-uint3 __ovld ctz(uint3 x);
-int4 __ovld ctz(int4 x);
-uint4 __ovld ctz(uint4 x);
-int8 __ovld ctz(int8 x);
-uint8 __ovld ctz(uint8 x);
-int16 __ovld ctz(int16 x);
-uint16 __ovld ctz(uint16 x);
-long __ovld ctz(long x);
-ulong __ovld ctz(ulong x);
-long2 __ovld ctz(long2 x);
-ulong2 __ovld ctz(ulong2 x);
-long3 __ovld ctz(long3 x);
-ulong3 __ovld ctz(ulong3 x);
-long4 __ovld ctz(long4 x);
-ulong4 __ovld ctz(ulong4 x);
-long8 __ovld ctz(long8 x);
-ulong8 __ovld ctz(ulong8 x);
-long16 __ovld ctz(long16 x);
-ulong16 __ovld ctz(ulong16 x);
+char __ovld __cnfn ctz(char x);
+uchar __ovld __cnfn ctz(uchar x);
+char2 __ovld __cnfn ctz(char2 x);
+uchar2 __ovld __cnfn ctz(uchar2 x);
+char3 __ovld __cnfn ctz(char3 x);
+uchar3 __ovld __cnfn ctz(uchar3 x);
+char4 __ovld __cnfn ctz(char4 x);
+uchar4 __ovld __cnfn ctz(uchar4 x);
+char8 __ovld __cnfn ctz(char8 x);
+uchar8 __ovld __cnfn ctz(uchar8 x);
+char16 __ovld __cnfn ctz(char16 x);
+uchar16 __ovld __cnfn ctz(uchar16 x);
+short __ovld __cnfn ctz(short x);
+ushort __ovld __cnfn ctz(ushort x);
+short2 __ovld __cnfn ctz(short2 x);
+ushort2 __ovld __cnfn ctz(ushort2 x);
+short3 __ovld __cnfn ctz(short3 x);
+ushort3 __ovld __cnfn ctz(ushort3 x);
+short4 __ovld __cnfn ctz(short4 x);
+ushort4 __ovld __cnfn ctz(ushort4 x);
+short8 __ovld __cnfn ctz(short8 x);
+ushort8 __ovld __cnfn ctz(ushort8 x);
+short16 __ovld __cnfn ctz(short16 x);
+ushort16 __ovld __cnfn ctz(ushort16 x);
+int __ovld __cnfn ctz(int x);
+uint __ovld __cnfn ctz(uint x);
+int2 __ovld __cnfn ctz(int2 x);
+uint2 __ovld __cnfn ctz(uint2 x);
+int3 __ovld __cnfn ctz(int3 x);
+uint3 __ovld __cnfn ctz(uint3 x);
+int4 __ovld __cnfn ctz(int4 x);
+uint4 __ovld __cnfn ctz(uint4 x);
+int8 __ovld __cnfn ctz(int8 x);
+uint8 __ovld __cnfn ctz(uint8 x);
+int16 __ovld __cnfn ctz(int16 x);
+uint16 __ovld __cnfn ctz(uint16 x);
+long __ovld __cnfn ctz(long x);
+ulong __ovld __cnfn ctz(ulong x);
+long2 __ovld __cnfn ctz(long2 x);
+ulong2 __ovld __cnfn ctz(ulong2 x);
+long3 __ovld __cnfn ctz(long3 x);
+ulong3 __ovld __cnfn ctz(ulong3 x);
+long4 __ovld __cnfn ctz(long4 x);
+ulong4 __ovld __cnfn ctz(ulong4 x);
+long8 __ovld __cnfn ctz(long8 x);
+ulong8 __ovld __cnfn ctz(ulong8 x);
+long16 __ovld __cnfn ctz(long16 x);
+ulong16 __ovld __cnfn ctz(ulong16 x);
 #endif //defined(__OPENCL_CPP_VERSION__) || (__OPENCL_C_VERSION__ >= CL_VERSION_2_0)
 
 /**
  * Returns mul_hi(a, b) + c.
  */
 char __ovld __cnfn mad_hi(char a, char b, char c);
 uchar __ovld __cnfn mad_hi(uchar a, uchar b, uchar c);
 char2 __ovld __cnfn mad_hi(char2 a, char2 b, char2 c);

clang/lib/Sema/OpenCLBuiltins.td

foreach name = ["abs"] in {
   def : Builtin<name, [AI2UGenTypeN, AIGenTypeN], Attr.Const>;
 }
 def : Builtin<"clz", [AIGenTypeN, AIGenTypeN], Attr.Const>;
 let MinVersion = CL12 in {
   def : Builtin<"popcount", [AIGenTypeN, AIGenTypeN], Attr.Const>;
 }
 let MinVersion = CL20 in {
   foreach name = ["ctz"] in {
-    def : Builtin<name, [AIGenTypeN, AIGenTypeN]>;
+    def : Builtin<name, [AIGenTypeN, AIGenTypeN], Attr.Const>;
   }
 }
 
 // --- 2 arguments ---
 foreach name = ["abs_diff"] in {
   def : Builtin<name, [AI2UGenTypeN, AIGenTypeN, AIGenTypeN], Attr.Const>;
 }
 foreach name = ["add_sat", "hadd", "rhadd", "mul_hi", "rotate", "sub_sat"] in {