[mlir][llvm] Fix export of 64-bit integer function attributes

[Mogball]

Summary

The allocsize attribute is weird because it packs two 32-bit values
into a 64-bit value. It also turns out that the passthrough attribute
exporter was using int, which is incorrectly handling 64-bit integers.

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp

if (kind == llvm::Attribute::None) {
     llvmFunc->addFnAttr(key, value);
     return success();
   }
 
   if (llvm::Attribute::isIntAttrKind(kind)) {
     if (value.empty())
       return emitError(loc) << "LLVM attribute '" << key << "' expects a value";
 
-    int result;
+    int64_t result;
     if (!value.getAsInteger(/*Radix=*/0, result))
       llvmFunc->addFnAttr(
           llvm::Attribute::get(llvmFunc->getContext(), kind, result));
     else
       llvmFunc->addFnAttr(key, value);
     return success();
   }
 

mlir/test/Target/LLVMIR/llvmir.mlir

 // CHECK: attributes #[[ATTR_GROUP]] = {
 // CHECK-DAG: noinline
 // CHECK-DAG: alignstack=4
 // CHECK-DAG: null_pointer_is_valid
 // CHECK-DAG: "foo"="bar"
 
 // -----
 
+// CHECK-LABEL: @my_allocator
+// CHECK: #[[ALLOC_ATTRS:[0-9]*]]
+llvm.func @my_allocator(i64) attributes {passthrough = [["allocsize", "4294967295"]]}
+
+// CHECK: attributes #[[ALLOC_ATTRS]] = {
+// CHECK-DAG: allocsize(0)
+
+// -----
+
 // CHECK-LABEL: @functionEntryCount
 // CHECK-SAME: !prof ![[PROF_ID:[0-9]*]]
 llvm.func @functionEntryCount() attributes {function_entry_count = 4242 : i64} {
   llvm.return
 }
 
 // CHECK: ![[PROF_ID]] = !{!"function_entry_count", i64 4242}