[clang] [clang] Fix --entry command line option (PR #69114)

[Fangrui Song via cfe-commits]

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@@ -0,0 +1,5 @@
+// RUN: %clang -### --entry test %s 2>&1 | FileCheck %s
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MaskRay wrote:

In the absence of `--target=`, the default target triple `LLVM_DEFAULT_TARGET_TRIPLE` is used. The selected `ToolChain` may not run AddLinkerInputs to render `-e`.

We should add `--target=` (common choice is a linux triple) and try reusing an existing test https://maskray.me/blog/2021-08-08-toolchain-testing#i-dont-know-an-existing-test-can-be-enhanced

Perhaps linker-opts.c or Xlinker-args.c

https://github.com/llvm/llvm-project/pull/69114