This reduces visual noise ("./foo.h" -> "foo.h") in error messages.
Fix all tests which fail as a result.
This also fixes a class of failures when FILE is used in modules, and
the module specifies 'textual header "foo.h"' but Clang has "./foo.h"
included elsewhere.
Finally this should fix Windows failure from https://reviews.llvm.org/D121658
Don't specify sizes of arrays explicitly. This is error prone.