[Issue 19395] New: extern(C++) mangling: posix mangling does not support `S_` token

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19395

          Issue ID: 19395
           Summary: extern(C++) mangling: posix mangling does not support
                    `S_` token
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: major
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: turkeyman at gmail.com

Posix mangling uses an `S_`, `S0_`, `S1_`, ... token to repeat the prior
element in the mangled name.

Without supporting this, we fail to mangle functions with repeated arguments:

  void fun(wchar x, wchar y, bool b); <-- __Z3funDsS_b  (ie; Ds is repeated
once)

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