Add missing nearbyintf16 builtin functions

[joshua-arch1]

Summary

To generate FROUND.H instructions in https://reviews.llvm.org/D143982, we need to use llvm intrinsics in IR files. Now I add corresponding nearbyintf16 builtin function to make sure these roundtointegral instructions can be generated from C codes.

[efriedma]

Can you add a codegen testcase? Otherwise LGTM.

[aaron.ballman]

Please also add a release note and add documentation for the builtin.

clang/include/clang/Basic/Builtins.def

 BUILTIN(__builtin_lrintl, "LiLd", "Fne")
 BUILTIN(__builtin_lrintf128, "LiLLd", "Fne")
 BUILTIN(__builtin_lround , "Lid", "Fne")
 BUILTIN(__builtin_lroundf, "Lif", "Fne")
 BUILTIN(__builtin_lroundl, "LiLd", "Fne")
 BUILTIN(__builtin_lroundf128, "LiLLd", "Fne")
 BUILTIN(__builtin_nearbyint , "dd", "Fnc")
 BUILTIN(__builtin_nearbyintf, "ff", "Fnc")
+BUILTIN(__builtin_nearbyintf16, "hh"  , "Fnc")
 BUILTIN(__builtin_nearbyintl, "LdLd", "Fnc")
 BUILTIN(__builtin_nearbyintf128, "LLdLLd", "Fnc")
 BUILTIN(__builtin_nextafter , "ddd", "Fne")
 BUILTIN(__builtin_nextafterf, "fff", "Fne")
 BUILTIN(__builtin_nextafterl, "LdLdLd", "Fne")
 BUILTIN(__builtin_nextafterf128, "LLdLLdLLd", "Fne")
 BUILTIN(__builtin_nexttoward , "ddLd", "Fne")
 BUILTIN(__builtin_nexttowardf, "ffLd", "Fne")

clang/lib/CodeGen/CGBuiltin.cpp

case Builtin::BI__builtin_log2f128:
                                    Intrinsic::log2,
                                    Intrinsic::experimental_constrained_log2));
 
     case Builtin::BInearbyint:
     case Builtin::BInearbyintf:
     case Builtin::BInearbyintl:
     case Builtin::BI__builtin_nearbyint:
     case Builtin::BI__builtin_nearbyintf:
+    case Builtin::BI__builtin_nearbyintf16:
     case Builtin::BI__builtin_nearbyintl:
     case Builtin::BI__builtin_nearbyintf128:
       return RValue::get(emitUnaryMaybeConstrainedFPBuiltin(*this, E,
                                 Intrinsic::nearbyint,
                                 Intrinsic::experimental_constrained_nearbyint));
 
     case Builtin::BIpow:
     case Builtin::BIpowf:

clang/test/CodeGenOpenCL/builtins-f16.cl

void test_half_builtins(half h0, half h1, half h2) {
   res = __builtin_log10f16(h0);
 
   // CHECK: call half @llvm.log2.f16(half %h0)
   res = __builtin_log2f16(h0);
 
   // CHECK: call half @llvm.log.f16(half %h0)
   res = __builtin_logf16(h0);
 
+  // CHECK: call half @llvm.nearbyint.f16(half %h0)
+  res = __builtin_nearbyintf16(h0);
+
   // CHECK: call half @llvm.rint.f16(half %h0)
   res = __builtin_rintf16(h0);
 
   // CHECK: call half @llvm.round.f16(half %h0)
   res = __builtin_roundf16(h0);
 
   // CHECK: call half @llvm.sin.f16(half %h0)
   res = __builtin_sinf16(h0);