Set capabilities for BuiltIn Decorations

BuiltIn Decorations have their own Enabling Capabilities, so report
those capabilities when emitting a BuiltIn Decoration.

Author: svenvh

llvm-spirv/lib/SPIRV/libSPIRV/SPIRVDecorate.h

@@ -81,7 +81,16 @@ class SPIRVDecorateGeneric : public SPIRVAnnotationGeneric {
   void setOwner(SPIRVDecorationGroup *Owner) { this->Owner = Owner; }
 
   SPIRVCapVec getRequiredCapability() const override {
-    return getCapability(Dec);
+    switch (Dec) {
+    case DecorationBuiltIn: {
+      // Return the BuiltIn's capabilities.
+      BuiltIn BI = static_cast<BuiltIn>(Literals.back());
+      return getCapability(BI);
+    }
+
+    default:
+      return getCapability(Dec);
+    }
   }
 
   SPIRVWord getRequiredSPIRVVersion() const override {

llvm-spirv/test/transcoding/OpenCL/sub_group_mask.cl

@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 %s -triple spir -cl-std=CL2.0 -emit-llvm-bc -o %t.bc
+
+// RUN: llvm-spirv %t.bc -o %t.spv
+// RUN: spirv-val %t.spv
+// RUN: llvm-spirv %t.spv -to-text -o - | FileCheck %s --check-prefix=CHECK-SPIRV
+// RUN: llvm-spirv %t.spv -r -o - | llvm-dis -o - | FileCheck %s --check-prefix=CHECK-LLVM
+
+// Taken from clang/lib/Headers/opencl-c{,-base}.h
+// TODO: remove these and switch to -fdeclare-opencl-builtins
+typedef unsigned int uint4 __attribute__((ext_vector_type(4)));
+uint4 __attribute__((overloadable)) get_sub_group_gt_mask(void);
+
+// CHECK-SPIRV: Capability GroupNonUniformBallot
+// CHECK-SPIRV: Decorate {{[0-9]+}} BuiltIn 4418
+
+// CHECK-LLVM: test_mask
+// CHECK-LLVM: call spir_func <4 x i32> @_Z21get_sub_group_gt_maskv()
+
+kernel void test_mask(global uint4 *out)
+{
+  *out = get_sub_group_gt_mask();
+}