[clang] [Clang][Cygwin] Use correct mangling rule (PR #158404)

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jeremyd2019 wrote:

The description could use some improvement.  How about something like

> In 45ca613c135ea7b5fbc63bff003f20bf20f62081, whether to mangle names based on calling conventions according to Microsoft conventions was refactored to a bool in the TargetInfo.  Cygwin targets also require this mangling, but were missed, presumably due to lack of test coverage of these targets.  This commit enables the name mangling for Cygwin, and also enables test coverage of this mangling on Cygwin targets.

https://github.com/llvm/llvm-project/pull/158404