[llvm] [ORC-RT] Initial check-in for a new, top-level ORC runtime project. (PR #113499)

[Lang Hames via llvm-commits]

lhames wrote:

@llvm-beanz @petrhosek I'm just getting back to this.

I've hit a sticking point on Darwin: If I want to do a multiconfig build and take the compiler-rt approach:
```bash
% xcrun cmake -GNinja -DCMAKE_BUILD_TYPE=Debug \
    -DLLVM_ENABLE_PROJECTS="llvm;clang" \
    -DLLVM_ENABLE_RUNTIMES="orc-rt" \
    -DDARWIN_macosx_ARCHS="arm64;x86-64" \
    /path/to/llvm-project/llvm
```
I still only get a single-architecture archive, presumably because the CMake in this PR doesn't speak Apple-multiarch (by design).

On the other hand if I try the Linux approach and specify:
```bash
% xcrun cmake -GNinja -DCMAKE_BUILD_TYPE=Debug \
    -DLLVM_ENABLE_PROJECTS="llvm;clang" \
    -DLLVM_ENABLE_RUNTIMES="orc-rt" \
    -DLLVM_RUNTIME_TARGETS="arm64-apple-darwin;x86_64-apple-darwin" \
    /Users/lhames/Projects/llvm/llvm-github/llvm
```
I get an error at config time:
```
CMake Error at runtimes/CMakeLists.txt:64 (message):
  compiler-rt for Darwin builds for all platforms and architectures using a
  single configuration.  Specify only a single darwin triple (e.g.
  x86_64-apple-darwin) in your targets list (and not a triple for a specific
  platform such as macos).  You can use variables such as
  COMPILER_RT_ENABLE_IOS and DARWIN_ios_ARCHS to control the specific
  platforms and architectures to build.  Set RUNTIMES_BUILD_ALLOW_DARWIN to
  allow a single darwin triple.
Call Stack (most recent call first):
  runtimes/CMakeLists.txt:623 (check_apple_target)
 ```
 Passing `-DRUNTIMES_BUILD_ALLOW_DARWIN` does allow a single triple to be specified, but doesn't allow multi-config.
 
 If building compiler-rt is required for multi-config builds I guess we're going to need some sort of interim solution for Darwin that allows existing projects (compiler-rt in particular) to keep building, while allowing new projects to use `-DLLVM_RUNTIME_TARGETS` on Apple. Would it make sense to add an `LLVM_RUNTIMES_ISOLATE_APPLE_LEGACY_BUILDS` option that resets `RUNTIMES_BUILD_ALLOW_DARWIN` (and any other arguments that aren't expected to be used for Apple-style builds) for Apple-style subprojects?

https://github.com/llvm/llvm-project/pull/113499