Tests where the RUN-lines/CHECK-ed output refer to line numbers in the test
file are a maintenance burden, as inserting text in the appropriate place
invalidates all the subsequent line numbers.

Lit supports %(line+n) for this, and FileCheck supports [[@LINE+N]].
But many existing tests don't make use of it and still need to be modified.

This commit adds a script that can find line numbers in tests according to a
regex and replace them with the appropriate relative-line reference.
It contains some options to avoid inappropriately rewriting tests where absolute
numbers are appropriate: a "nearby" threshold and a refusal by default to
replace only some matched line numbers.

I've applied it to CodeComplete tests, this proves the concept but also are the
single worst group of tests I've seen in this respect.
These changes are likely to hit merge conflicts, but can be regenerated with:

```
find ../clang/test/CodeCompletion/ -type f | grep -v /Inputs/ | xargs ../llvm/utils/relative_lines.py --verbose --near=20 --pattern='-code-completion-at[ =]%s:(\\d+):' --pattern='requires fix-it: {(\d+):\d+-(\d+):\d+}'
````

As requested in https://reviews.llvm.org/D140044

Fixes https://github.com/llvm/llvm-project/issues/59553

Differential Revision: https://reviews.llvm.org/D140217

Summary:
This patch removes IdentifierInfo from completion token after remembering
the identifier in the preprocessor.

Prior to this patch, completion token had the IdentifierInfo set to null when
completing at the start of identifier and to the II for completion prefix
when in the middle of identifier.

This patch unifies how code completion token is handled when it is insterted
before the identifier and in the middle of the identifier.

The actual IdentifierInfo can still be obtained from the Preprocessor.

Reviewers: bkramer, arphaman

Reviewed By: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42241

llvm-svn: 323133