[mlir][spirv] Fix coop matrix getExtension.

[ThomasRaoux]

Summary

Stack variable was being used beyond its lifetime.

[antiagainst]

Ah yes! Good catch on this one. We are trying to avoid allocating SmallVectors of SmallVectors. so SmallVectors of ArrayRefs. It does mean that the lifetime management become more manual..

mlir/lib/Dialect/SPIRV/SPIRVTypes.cpp

 unsigned CooperativeMatrixNVType::getColumns() const {
   return getImpl()->columns;
 }
 
 void CooperativeMatrixNVType::getExtensions(
     SPIRVType::ExtensionArrayRefVector &extensions,
     Optional<StorageClass> storage) {
   getElementType().cast<SPIRVType>().getExtensions(extensions, storage);
-  extensions.push_back(Extension::SPV_NV_cooperative_matrix);
+  static const Extension exts[] = {Extension::SPV_NV_cooperative_matrix};
+  ArrayRef<Extension> ref(exts, llvm::array_lengthof(exts));
+  extensions.push_back(ref);
 }
 
 void CooperativeMatrixNVType::getCapabilities(
     SPIRVType::CapabilityArrayRefVector &capabilities,
     Optional<StorageClass> storage) {
   getElementType().cast<SPIRVType>().getCapabilities(capabilities, storage);
-  capabilities.push_back(Capability::CooperativeMatrixNV);
+  static const Capability caps[] = {Capability::CooperativeMatrixNV};
+  ArrayRef<Capability> ref(caps, llvm::array_lengthof(caps));
+  capabilities.push_back(ref);
 }
 
 //===----------------------------------------------------------------------===//
 // ImageType
 //===----------------------------------------------------------------------===//
 
 template <typename T> static constexpr unsigned getNumBits() { return 0; }
 template <> constexpr unsigned getNumBits<Dim>() {