[compiler-rt] [compiler-rt] Don't redefine LLVM_COMMON_CMAKE_UTILS if it's defined (PR #66761)

[Xi Ruoyao via llvm-commits]

xry111 wrote:

> > If we are building compiler-rt inside llvm-x.y.z.src directory, we should not redefine LLVM_COMMON_CMAKE_UTILS.
> 
> Can you add more instructions?
> 
> I can see that some subprojects checks `DEFINED` while libcxx/ doesn't check whether `LLVM_COMMON_CMAKE_UTILS` is defined

Extracting compiler-rt into llvm/projects/ and build:

```
tar xf llvm-17.0.1.src.tar.xz
tar xf cmake-17.0.1.src.tar.xz
mv cmake-17.0.1.src cmake
tar xf third-party-17.0.1.src.tar.xz
mv third-party-17.0.1.src third-party
tar xf compiler-rt-17.0.1.src.tar.xz
mv compiler-rt-17.0.1.src llvm-17.0.1.src/projects/compiler-rt
mkdir llvm-17.0.1.src/build
cd llvm-17.0.1.src/build
cmake ..
```

https://github.com/llvm/llvm-project/pull/66761