This flag was introduced for clang 9, but we use it in the `lldClang`
wrapper for `llvmPackages` 7, 8 and 9. For this purpose the patch was
backported for `llvmPackages_8.clang`, but not for `llvmPackages_7.clang`
which has been done in this commit.
`lldClang` is mostly used when cross compiling and
`stdenv.hostPlatform.useLLVM` is true. Most likely this problem wasn't
noticed since `useLLVM` with `llvmPackages_7` was broken for other
reasons as well and all cross targets (like `wasi32`) which have
`useLLVM` at the moment use `llvmPackages_8`.
With this change tests.cross.llvm.hello.{musl64, …} works again.
