[clang] Fixes inf loop parsing fixed point literal (#83071)

Clang was incorrectly finding the start of the exponent in a fixed point
hex literal. It would unconditionally find the first `e/E/p/P` in a
constant regardless of if it were hex or not and parser the remaining
digits as an APInt. In a debug build, this would be caught by an
assertion, but in a release build, the assertion is removed and we'd end
up in an infinite loop.

Fixes #83050