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

[Fangrui Song 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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MaskRay wrote:

This is the first time clang/test/Driver has a `*-linux-llvm` triple test. I expect a comment that this is about a toolchain using llvm libc. 

There are many users using linux toolchains without using llvm libc, so I was a -1 on the original `linux-llvm` naming.

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