[GWP-ASan] Fix up bad report for in-page underflow w/ UaF

Complex scenario, but reports when there's both a use-after-free and
buffer-underflow that is in-page (i.e. doesn't touch the guard page)
ended up generating a pretty bad report:

'Use After Free at 0x7ff392e88fef (18446744073709551615 bytes into a
1-byte allocation at 0x7ff392e88ff0) by thread 3836722 here:'

(note the 2^64-bytes-into-alloc, very cool and good!)

Fix up that case, and add a diagnostic about when you have both a
use-after-free and a buffer-overflow that it's probably a bogus report
(assuming the developer didn't *really* screw up and have a uaf+overflow
bug at the same time).

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D139885