Fix UB in UntypedPtrTyMap (#3283)

SmallDenseMap requires sentinel values out of the
`SPIRVStorageClassKind` range. This became a build error after
llvm-project commit `38f82534bbe9 ("Reject out-of-bounds enum sentinels
in DenseMap/DenseSet. (#150308)", 2025-07-24)`.

As the `SPIRVStorageClassKind` enum comes from an external header we
cannot change it to an enum class or add an underlying type. This is
actually solved in the `spirv.hpp11` header, but migrating to that
header requires more widespread changes; just switch to a
`std::unordered_map` for now.

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@db0ec312a9cacf9

Author: svenvh

llvm-spirv/lib/SPIRV/libSPIRV/SPIRVModule.cpp

@@ -634,7 +634,7 @@ class SPIRVModuleImpl : public SPIRVModule {
   SPIRVUnknownStructFieldMap UnknownStructFieldMap;
   SPIRVTypeBool *BoolTy;
   SPIRVTypeVoid *VoidTy;
-  SmallDenseMap<SPIRVStorageClassKind, SPIRVTypeUntypedPointerKHR *>
+  std::unordered_map<SPIRVStorageClassKind, SPIRVTypeUntypedPointerKHR *>
       UntypedPtrTyMap;
   SmallDenseMap<unsigned, SPIRVTypeInt *, 4> IntTypeMap;
   SmallDenseMap<std::pair<unsigned, unsigned>, SPIRVTypeFloat *, 4>