[mlir] fix dereferencing of optional sym_name attribute

[ashay-github]

Summary

sym_name is an optional attribute of ModuleOp, so it is unsafe to
fetch the underlying value without checking whether it is non-empty.
Such unsafe dereferencing causes the lower-host-to-llvm-calls_fail.mlir
test to segfault. Although this bug existed for four months, it wasn't
triggered, since previous tests executed a code path that used a default
value instead of one fetched from the module attribute.

This patch makes the code use a default value if the optional attribute
does not have a value.

[mehdi_amini]

Can you add a test for this please?

[ashay-github]

Added unit tests to check that lowering using either the specified or the
default attribute value.

mlir/lib/Conversion/SPIRVToLLVM/ConvertLaunchFuncToLLVMCalls.cpp

if (hasDescriptorSetAndBinding(globalOp))
       globalVariableMap[calculateGlobalIndex(globalOp)] = globalOp;
   }
   return success();
 }
 
 /// Encodes the SPIR-V module's symbolic name into the name of the entry point
 /// function.
 static LogicalResult encodeKernelName(spirv::ModuleOp module) {
-  StringRef spvModuleName = *module.getSymName();
+  StringRef spvModuleName = module.getSymName().value_or(kSPIRVModule);
   // We already know that the module contains exactly one entry point function
   // based on `getKernelGlobalVariables()` call. Update this function's name
   // to:
   //   {spv_module_name}_{function_name}
   auto entryPoints = module.getOps<spirv::EntryPointOp>();
   if (!llvm::hasSingleElement(entryPoints)) {
     return module.emitError(
         "The module must contain exactly one entry point function");