clang incorrectly expands trigraphs when pasting tokens

This program prints `|`, but it should print `??!`.

```c
#include

#define PASTE(X, Y) X ## Y

int main(void) {
printf("%ls\n", PASTE(L, "??\
!"));
}
```

After pasting the tokens `L` and `"??!"`, clang incorrectly expands the trigraph inside of the second token.

https://godbolt.org/z/s7esnTdjz

Start by compiling and running the provided program, then inspect clang's preprocessor handling of trigraphs during token pasting. Trace the handling of the pasted L and string tokens and add a regression test covering the example, with completion shown by the program producing the expected ??! output.

Written by the indexing model from the issue text.