[Offload] Deprecate openmp and offload projects builds (#136314)

Summary:
These are compiler runtimes are currently have weird support for
projects / runtimes. We should depcrate this to where the only expected
configuration is via a runtimes build. If the user wants a custom setup
they can still use the standalone build.

I remember there were some concerens around the OpenMP module file for
`flang`. Were those resolved?

Fixes: #124014

Author: jhuber6

llvm/CMakeLists.txt

@@ -164,6 +164,20 @@ if ("compiler-rt" IN_LIST LLVM_ENABLE_PROJECTS)
     "https://compiler-rt.llvm.org/ for building the runtimes.")
 endif()
 
+if ("offload" IN_LIST LLVM_ENABLE_PROJECTS)
+  message(WARNING "Using LLVM_ENABLE_PROJECTS=offload is deprecated now, and will "
+    "become a fatal error in the LLVM 21 release.  Please use "
+    "-DLLVM_ENABLE_RUNTIMES=offload or see the instructions at "
+    "https://openmp.llvm.org/ for building the runtimes.")
+endif()
+
+if ("openmp" IN_LIST LLVM_ENABLE_PROJECTS)
+  message(WARNING "Using LLVM_ENABLE_PROJECTS=openmp is deprecated now, and will "
+    "become a fatal error in the LLVM 21 release.  Please use "
+    "-DLLVM_ENABLE_RUNTIMES=openmp or see the instructions at "
+    "https://openmp.llvm.org/ for building the runtimes.")
+endif()
+
 if ("flang-rt" IN_LIST LLVM_ENABLE_RUNTIMES)
   if (NOT "flang" IN_LIST LLVM_ENABLE_PROJECTS)
     message(FATAL_ERROR "Flang is not enabled, but is required for the Flang-RT runtime")