[llvm-dev] [RFC] Handling cmake policies

[Don Hinton via llvm-dev]

Currently, cmake policies are manually maintained by each project.  This is
done via a set of `cmake_policy()` commands following the initial
`cmake_minimum_required()` command at the top of each CMakeLists.txt
project file.

Somewhat surprisingly, these sets are local to each project and independent
of each other -- even for in-tree builds containing multiple
sub-projects.  This
is because cmake policies are reset whenever `cmake_minimum_required()` is
called, which is the first command in each of our CMakeLists.txt project
files.

In order to solve this problem, as well as improve maintainability, I'd
like to propose the following:

* For all projects except llvm, move `cmake_minimum_required()` inside the
standalone build if-branch, i.e., `if(CMAKE_SOURCE_DIR STREQUAL
CMAKE_CURRENT_SOURCE_DIR)`, at the top of the CMakeLists.txt project file.
This will prevent unnecessary calls `cmake_minimum_required()` for in-tree
builds, and allow all sub-projects to use the same policies set by llvm.

* Move all `cmake_policy()` commands out of llvm/CMakeLists.txt and into a
separate file, e.g., AddLLVM.cmake, which is always included in each
project.  This will guarantee all projects use the same set of policies
even when built as a standalone or external project, e.g., runtimes.

Sub-projects can still override policies as needed by setting them after
including AddLLVM.cmake, or by using `cmake_policy(PUSH|POP)` for local
changes.
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