[clang] [flang] [flang][driver] Make -stdlib= option visible to flang and silently ignored by it (PR #110598)

[Tom Eccles via cfe-commits]

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+// REQUIRES: system-linux
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tblah wrote:

I'm not sure about this test. Usually we would go about testing something like this by running FileCheck on the output of `flang-new -###` to see what is passed to the linker (and that the linker gets called at all instead of us generating an error for an invalid argument). 

https://github.com/llvm/llvm-project/pull/110598