Fix bugs when an included file name is typo corrected.

D52774 fixed a bug with typo correction of includes, but didn't add
a test.

D65907 then broke recovery of typo correction of includes again,
because it extracted the code that writes to Filename to a separate
function that took the parameter not by reference.

Fix that, and also don't repeat the slash normalization computation
and fix both lookup and regular file name after recovery.

Differential Revision: https://reviews.llvm.org/D79595