[SystemZ] constexpr variable promoted to global does not minimum alignment

```cpp
char test(unsigned n) {
constexpr char digits[] = "foobar";
return digits[n];
}
```
`clang --target=s390x-unknown-linux-gnu -S -emit-llvm -o - -O2 test.cpp` produces:
```llvm
@__const._Z4testj.digits = private unnamed_addr constant [7 x i8] c"foobar\00", align 1
```
Because it does not have the minimum global alignment of 2, it gets lowered to this GOT reference:
```
lgrl %r1, .L__const._Z4testj.digits@GOT
```

This is probably not supposed to happen? I expect either Clang should emit this with align 2, or the backend should raise the alignment (it has private linkage), or both.

Reproduce the issue with the provided clang --target=s390x-unknown-linux-gnu -S -emit-llvm -O2 command and inspect the emitted global alignment and subsequent SystemZ GOT lowering. Trace whether Clang's constexpr promotion or the backend handles alignment, then verify that the private constant receives the required minimum alignment or avoids the unnecessary GOT reference.

Written by the indexing model from the issue text.