[Bug c++/105878] New: Error message for const usage inside a capture should be improved

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105878

            Bug ID: 105878
           Summary: Error message for const usage inside a capture should
                    be improved
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
void f(int a)
{
  [& const a](){};
}
```
The current error message is:
```
<source>: In function 'void f(int)':
<source>:3:5: error: expected ',' before 'const'
    3 |   [& const a](){};
      |     ^~~~~~
      |     ,
<source>:3:6: error: expected identifier before 'const'
    3 |   [& const a](){};
      |      ^~~~~
```
clang gives:
```
<source>:3:6: error: expected variable name or 'this' in lambda capture list
  [& const a](){};
     ^
```
Which is slightly better.
but I think we could do better and say qualifers should not be in the capture
list.