[clang] [clang] Fix underlying type of EmbedExpr (PR #99050)

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

> If you have an embed file with a single byte in it whose value is 0xFF, you could use a test like:
struct S {
  signed char ch;
};
constexpr struct S s = {
#embed "foo"
};
which should give the same behavior as: https://godbolt.org/z/Tno5YPexM

Thanks! That worked.

I didn't add these though
```
static_assert(_Generic(
#embed __FILE__ limit(1)
  , int : 1, default : 0));

static_assert(alignof(typeof(
#embed __FILE__ limit(1)
)) == alignof(int));

printf("%hhu", // Do we get a -Wformat diagnostic about the specifier mismatch?
#embed __FILE__ limit(1)
);
```
for several reasons:

- They don't test this patch as it affects only what happens in an initializer list
- The one with alignof/typeof asserts for C++ (not for C) and I like to make a fix and add this test in a separate PR
- we don't emit warnings even for `printf("%hhu", 300);`

https://github.com/llvm/llvm-project/pull/99050