llvm: introduce [build/link]_llvm_dylib (#27450)

Apart from building a single dylib for LLVM, users should also be able
to link tools against it.

share/spack/gitlab/cloud_pipelines/stacks/e4s-on-power/spack.yaml

@@ -119,7 +119,7 @@ spack:
     - libnrm
     - libquo
     - libunwind
-    - llvm +all_targets +clang +compiler-rt +libcxx +lld +lldb +llvm_dylib +flang ~cuda
+    - llvm +all_targets +clang +compiler-rt +libcxx +lld +lldb +build_llvm_dylib +flang ~cuda
     - loki
     - mercury
     - metall

share/spack/gitlab/cloud_pipelines/stacks/e4s/spack.yaml

@@ -130,7 +130,7 @@ spack:
     - libnrm
     - libquo
     - libunwind
-    - llvm +all_targets +clang +compiler-rt +libcxx +lld +lldb +llvm_dylib +flang ~cuda
+    - llvm +all_targets +clang +compiler-rt +libcxx +lld +lldb +build_llvm_dylib +flang ~cuda
     - loki
     - mercury
     - metall

var/spack/repos/builtin/packages/llvm/package.py

@@ -123,11 +123,16 @@ class Llvm(CMakePackage, CudaPackage):
         "less memory to build, less stable",
     )
     variant(
-        "llvm_dylib",
+        "build_llvm_dylib",
         default=False,
         description="Build LLVM shared library, containing all "
         "components in a single shared library",
     )
+    variant(
+        "link_llvm_dylib",
+        default=False,
+        description="Link LLVM tools against the LLVM shared library",
+    )
     variant(
         "all_targets",
         default=False,
@@ -190,7 +195,8 @@ class Llvm(CMakePackage, CudaPackage):
     depends_on("gmp", when="@:3.6 +polly")
     depends_on("isl", when="@:3.6 +polly")
 
-    conflicts("+llvm_dylib", when="+shared_libs")
+    conflicts("+build_llvm_dylib", when="+shared_libs")
+    conflicts("+link_llvm_dylib", when="~build_llvm_dylib")
     conflicts("+lldb", when="~clang")
     conflicts("+libcxx", when="~clang")
     conflicts("+internal_unwind", when="~clang")
@@ -552,9 +558,12 @@ def cmake_args(self):
             projects.append("polly")
             cmake_args.append(define("LINK_POLLY_INTO_TOOLS", True))
 
-        cmake_args.append(from_variant("BUILD_SHARED_LIBS", "shared_libs"))
-        cmake_args.append(from_variant("LLVM_BUILD_LLVM_DYLIB", "llvm_dylib"))
-        cmake_args.append(from_variant("LLVM_USE_SPLIT_DWARF", "split_dwarf"))
+        cmake_args.extend([
+            from_variant("BUILD_SHARED_LIBS", "shared_libs"),
+            from_variant("LLVM_BUILD_LLVM_DYLIB", "build_llvm_dylib"),
+            from_variant("LLVM_LINK_LLVM_DYLIB", "link_llvm_dylib"),
+            from_variant("LLVM_USE_SPLIT_DWARF", "split_dwarf")
+        ])
 
         # By default on Linux, libc++.so is a linker script, and CMake tries to add the
         # CMAKE_INSTALL_RPATH to it, which fails, causing installation to fail. The