[FileCheck] Call out var captures on unmatched patterns (#206303)
This patch is motivated by an #llvm IRC chat in 2019 with @AaronBallman,
where he pointed out an example similar to following:
```
$ cat input
[[clang::optnone]] void foo() {
$ cat check
CHECK: [[clang::optnone]] void foo() {
$ FileCheck check < input |& tail -7
Input was:
<<<<<<
1: [[clang::optnone]] void foo() {
check:1'0 { } search range (exclusive bounds)
check:1'1 error: no match found
check:1'2 ? possible intended match
>>>>>>
```
It is very easy to miss why that fails. This patch adds one more note
that hopefully clears up the confusion:
```
$ FileCheck check < input |& tail -8
Input was:
<<<<<<
1: [[clang::optnone]] void foo() {
check:1'0 { } search range (exclusive bounds)
check:1'1 error: no match found
check:1'2 pattern attempts to capture variables: "clang"
check:1'3 ? possible intended match
>>>>>>
```
That is, where matching patterns print successful variable captures,
this patch makes unmatching patterns print attempted variable captures.
llvm-project
0f1f4562 - [FileCheck] Call out var captures on unmatched patterns (#206303)
