Make 'static assertion failed' diagnostics point to the static assertion expression

"static assertion failed" pointed to the static_assert token and
then underlined the static assertion expression:

<source>:3:1: error: static assertion failed
    static_assert(false);
    ^             ~~~~~
    1 error generated.
See Godbolt: https://godbolt.org/z/r38booz59

Now it points to and highlights the assertion expression.

Fixes https://github.com/llvm/llvm-project/issues/61951
Differential Revision: https://reviews.llvm.org/D147745