[clang] [FreeBSD] Support -stdlib=libstdc++ (PR #126302)

[Fangrui Song via cfe-commits]

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@@ -1,9 +1,13 @@
 // RUN: %clangxx %s -### -o %t.o --target=amd64-unknown-freebsd -stdlib=platform 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-DEFAULT %s
 // RUN: %clangxx %s -### -o %t.o --target=amd64-unknown-freebsd10.0 -stdlib=platform 2>&1 \
-// RUN:   | FileCheck --check-prefix=CHECK-TEN %s
+// RUN:   | FileCheck --check-prefix=CHECK-DEFAULT %s
+// RUN: %clangxx %s -### -o %t.o --target=amd64-unknown-freebsd -stdlib=libc++ 2>&1 \
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MaskRay wrote:

`-o %t.o` is unneeded and can be dropped. We use `-###`, so there is no output.

https://github.com/llvm/llvm-project/pull/126302