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

[Tom Eccles via cfe-commits]

tblah wrote:

I don't think it is correct to say that the option was ignored. It may still be read by the compiler driver (which generates the link line). In fact it probably *should* be processed by the compiler driver in order to set the C++ standard library correctly - otherwise you could just not set this in your `LDFLAGS`.

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