[ADT] Fix alignment check in unique_function constructor (#99403)

Right now the check fails for any state-capturing lambda since this
expression - `alignof(decltype(StorageUnion.InlineStorage))` - returns 1
for the alignment value and not 4/8 as expected
([MSVC|Clang|GCC](https://godbolt.org/z/eTEdq4xjM)). So this check fails
for pretty much any state-capturing callable we try to store into a
`unique_function` and we take the out-of-line storage path:

\llvm-project\llvm\include\llvm\ADT\FunctionExtras.h,
`UniqueFunctionBase` constructor (line ~266):

```
if (sizeof(CallableT) > InlineStorageSize ||
    alignof(CallableT) > alignof(decltype(StorageUnion.InlineStorage))) {
  // ...
}
```

The fix is simply to use an explicit const variable to store the
alignment value.

There is no easy way to unit-test the fix since inline storage is
considered to be an implementation detail so we shouldn't assume how the
lambda ends up being stored.