[cfe-dev] [FileCheck] RFC: Add support for line anchors.

[Nathan James via cfe-dev]

Hello,

I was wondering about extending FileCheck to enable creating line
anchors. These are numeric variables that hold the value of the line
number that where they were defined.

The motivation for this comes from test cases using clang-based
diagnostics which often include notes attached to source locations in
different parts of the file. In order to test for the correct location
of the note, the line number has to be written explicitly or as an,
often large, offset to the current line. This harms both readability
and maintainability. Using this new system one could append a line of
interest with an anchor-comment and refer back to it inside FileCheck.

I have created a basic patch that implements this here 
https://reviews.llvm.org/D84037 but it definitely needs a few looks
over by people who are more clued up on the internal of FileCheck.

The current syntax, based off this patch, is as follows:
  - Added a command line option called `anchor-prefix` which is a
comma-seperated list of prefixes to be used when declaring anchors.
This is defaulted to `LINE-ANCHOR`
  - To declare a anchor in the test file use
    `LINE-ANCHOR: ANCHOR_NAME`
    note: If you specify a different anchor-prefix using the command
line, use that name instead of `LINE-ANCHOR`
    ANCHOR_NAME Follows the rules all other variable names aside from
the fact it can't start with '$'.
  - When referring to an anchor in a check use the same numeric
variable syntax that FileCheck already supports:
    `CHECK: [[#ANCHOR_NAME]][[#ANCHOR_NAME+1]]`

Here is a brief (contrived) example of the usage of this:
```
#define BAD_FUNCTION() badFunction() // LINE-ANCHOR: BAD_FUNC
// Further down in the file
  BAD_FUNCTION();
CHECK-NOTES: [[@LINE-1]]:3: warning: called a bad function
CHECK-NOTES :[[#BAD-FUNC]]:3: note: expanded from macro 'BAD_FUNCTION'
```

Regards,
Nathan James