[clang] [Clang] Include LLVM libc header directory from Linux (PR #175593)

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,3 @@
+// RUN:   %clang -### --target=x86_64-unknown-linux-llvm --sysroot=%S/Inputs/basic_llvm_linux_tree \
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AaronBallman wrote:

FWIW, I still would prefer to get to a place where users can do something like `-cstdlib=libc` vs `-cstdlib=musl`, etc instead of having to muck about with the environment on the triple, so it's more similar to how you select the C++ standard library. But I recognize that's far outside the scope of this PR too.

https://github.com/llvm/llvm-project/pull/175593