[llvm] [Docs] Update contribution guide to remove the single-commit requirement (PR #212031)

[via llvm-commits]

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@@ -29,8 +29,10 @@ let people know you are working on it.
 Then try to reproduce and fix the bug with upstream LLVM. Start by building
 LLVM from source as described in {doc}`GettingStarted` and
 use the built binaries to reproduce the failure described in the bug. Use
-a debug build (`-DCMAKE_BUILD_TYPE=Debug`) or a build with assertions
-(`-DLLVM_ENABLE_ASSERTIONS=On`, enabled for Debug builds).
+a build with assertions (`-DLLVM_ENABLE_ASSERTIONS=On`). If you want to build
+LLVM in Debug mode, consider using `LLVM_PARALLEL_LINK_JOBS` and set the linker
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Sirraide wrote:

Did that; I also broke up this paragraph a bit, added a sentence that recommends passing `-DLLVM_UNREACHABLE_OPTIMIZE=OFF`, as well as a link to the 'Building LLVM w/ CMake' page because I figured that might be useful to have

https://github.com/llvm/llvm-project/pull/212031