[CMake] Enable CMP0179 when enabling CMP0156 to avoid LLD static library link order bug

This fixes the following linking failure for libclang-cpp.so with -DLLVM_LINK_LLVM_DYLIB=ON:
```
ld.lld: error: undefined symbol: llvm::omp::getOpenMPClauseName(llvm::omp::Clause)
>>> referenced by OpenMPKinds.cpp
>>>               tools/clang/lib/Basic/CMakeFiles/obj.clangBasic.dir/OpenMPKinds.cpp.o:(clang::getOpenMPSimpleClauseTypeName(llvm::omp::Clause, unsigned int))
>>> referenced by SemaOpenMP.cpp
>>>               tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaOpenMP.cpp.o:(clang::SemaOpenMP::CheckOMPRequiresDecl(clang::SourceLocation, llvm::ArrayRef<clang::OMPClause*>))
>>> referenced by SemaOpenMP.cpp
>>>               tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/SemaOpenMP.cpp.o:(clang::SemaOpenMP::CheckOMPRequiresDecl(clang::SourceLocation, llvm::ArrayRef<clang::OMPClause*>))
>>> referenced 166 more times

[tons more]
```

This is exhibited under the following conditions:
* LLVM LLD
* CMake 3.31.0
* CMP0156=NEW
* CMP0179=OLD

CMake 3.31's new linker architecture changes the library link order.
Per LLD's design documents it should be immune to such reodering like other popular linkers (gold, Apple's), but isn't.

A workaround for LLD is expected in CMake 3.31.1 but CMP0179=NEW changes the linking order such that the bug is avoided.
Deduplication is better in general so always enable it when possible.

See: https://gitlab.kitware.com/cmake/cmake/-/issues/26447
Fixes: cb90d5b

Author: tambry

cmake/Modules/CMakePolicy.cmake

@@ -37,4 +37,13 @@ endif()
 # building with the Apple linker.
 if(POLICY CMP0156)
   cmake_policy(SET CMP0156 NEW)
+
+  # CMP0179: De-duplication of static libraries on link lines keeps first occurrence.
+  # Dependent on CMP0156=NEW. Keeps the first occurrence of static libraries
+  # during deduplication to unify the behaviour on all platforms.
+  # Also works around LLD bug exhibited since CMake 3.31.0 with an expected
+  # workaround in 3.31.1 (https://gitlab.kitware.com/cmake/cmake/-/issues/26447).
+  if(POLICY CMP0179)
+    cmake_policy(SET CMP0179 NEW)
+  endif()
 endif()