[llvm-bugs] [Bug 38095] New: function attribute "diagnose_if" doesn't see const char *

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38095

            Bug ID: 38095
           Summary: function attribute "diagnose_if" doesn't see const
                    char *
           Product: clang
           Version: 6.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: naruse at airemix.jp
                CC: llvm-bugs at lists.llvm.org

Following code has two functions with "dignose_if" function attribute.
Their difference is just whose argument is `char *` or `const char *`.
But `const char *` one doesn't show error.
Is this intended behavior?

```
% clang --version
clang version 6.0.0 (tags/RELEASE_600/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
% cat b.c
int scan_args(char *fmt)
    __attribute__((diagnose_if(fmt[0]!=2,"invalid format string","error")))
{
    return 0;
}

int scan_args_const(const char *fmt)
    __attribute__((diagnose_if(fmt[0]!=2,"invalid format string","error")))
{
    return 0;
}

int
main(int argc, char **argv) {
    return scan_args("20") + scan_args_const("20");
}
```

Expected behavior:
```
%  clang b.c
b.c:15:26: error: invalid format string
    return scan_args("20") + scan_args_const("20");
                         ^
b.c:2:20: note: from 'diagnose_if' attribute on 'scan_args':
    __attribute__((diagnose_if(fmt[0]!=2,"invalid format string","error")))
                   ^           ~~~~~~~~~
b.c:15:50: error: invalid format string
    return scan_args("20") + scan_args_const("20");
                                                 ^
b.c:8:20: note: from 'diagnose_if' attribute on 'scan_args_const':
    __attribute__((diagnose_if(fmt[0]!=2,"invalid format string","error")))
                   ^           ~~~~~~~~~
2 errors generated.
```

Actual behavior:
```
%  clang b.c
b.c:15:26: error: invalid format string
    return scan_args("20") + scan_args_const("20");
                         ^
b.c:2:20: note: from 'diagnose_if' attribute on 'scan_args':
    __attribute__((diagnose_if(fmt[0]!=2,"invalid format string","error")))
                   ^           ~~~~~~~~~
1 error generated.
```

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