[llvm] [FileCheck]: Fix diagnostic for trailing CHECK-NOT (PR #78412)

[Thomas Preud'homme via llvm-commits]

RoboTux wrote:

That works when the prefix of the last CHECK-NOT correspond to the prefix of the failing trailing CHECK-NOT. It fails in the following case though:
<input>
placeholder1
placeholder2

<check file>
LEADING: placeholder1
MIDDLE-NOT: placeholder2
TRAILING-NOT: placeholder3

and FileCheck is called with --prefixes=LEADING,MIDDLE,TRAILING: it shows TRAILING-NOT even though it's the MIDDLE-NOT that fail.

Likewise if we have this instead with --implicit-check-not "placeholder2":

<input>
placeholder1
placeholder2

<check file>
LEADING: placeholder1
TRAILING-NOT: placeholder3

It would show TRAILING-NOT in the error message even though it's the inplicit check not that failed.

https://github.com/llvm/llvm-project/pull/78412