miscompile performing list initialization via constructor from embed expression

[Testcase](https://clang.godbolt.org/z/7nMshEa7f):
```c++
#include

struct X {
template
constexpr X(T ...t) : sum((... + int(t))), size(sizeof...(T)) {}
int sum;
int size;
} x = {
#embed __FILE__
};

int main() {
std::print("file size: {0}, content sum: {1}", x.size, x.sum);
}
```
reports
```console
file size: 1, content sum: 35
```
It looks like only the first element of the embed expression actually gets passed to the constructor (strangely we have an `EmbedExpr` of type `int` here covering more than one element of the file):
```clangast
|-VarDecl line:8:3 used x 'struct X' cinit external-linkage
| `-CXXConstructExpr 'struct X' 'void (int)' list
| `-EmbedExpr 'int'
| |-begin: 0
| `-number of elements: 262
```
We probably have to expand the embed to one expression per function argument here, [like we do for direct non-list initialization](https://clang.godbolt.org/z/v431aeTsx).

Start with the linked Compiler Explorer test case and compare its AST with the direct non-list initialization example. Trace how the EmbedExpr with multiple elements is handled when constructing X from a braced initializer. Done means each embedded element reaches the constructor as a separate argument and the reported size and sum match the file contents.

Written by the indexing model from the issue text.