Packaging: Virtual package for libllvm (#27200)

Add an abstraction around libllvm to allow libllvm
providers to be specified for all packages.

This is targeting allowing mesa to build against
llvm-amdgpu or intel-llvm or llvm or any other
custom llvm variant that arises for specific GPU
toolchains

etc/spack/defaults/packages.yaml

@@ -34,6 +34,7 @@ packages:
       java: [openjdk, jdk, ibm-java]
       jpeg: [libjpeg-turbo, libjpeg]
       lapack: [openblas, amdlibflame]
+      libllvm: [llvm, llvm-amdgpu]
       lua-lang: [lua, lua-luajit]
       mariadb-client: [mariadb-c-client, mariadb]
       mkl: [intel-mkl]

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

@@ -35,6 +35,16 @@ class LlvmAmdgpu(CMakePackage):
     variant('build_type', default='Release', values=("Release", "Debug", "RelWithDebInfo"), description='CMake build type')
     variant('rocm-device-libs', default=True, description='Build ROCm device libs as external LLVM project instead of a standalone spack package.')
     variant('openmp', default=True, description='Enable OpenMP')
+    variant(
+        "llvm_dylib",
+        default=False,
+        description="Build LLVM shared library, containing all "
+        "components in a single shared library",
+    )
+
+    provides('libllvm@11', when='@3.5:3.8')
+    provides('libllvm@12', when='@3.9:4.2')
+    provides('libllvm@13', when='@4.3:')
 
     depends_on('cmake@3.4.3:',  type='build', when='@:3.8')
     depends_on('cmake@3.13.4:', type='build', when='@3.9.0:')
@@ -131,6 +141,9 @@ def cmake_args(self):
                 self.define('LLVM_EXTERNAL_DEVICE_LIBS_SOURCE_DIR', dir)
             ])
 
+        if '+llvm_dylib' in self.spec:
+            cmake_args.append("-DLLVM_BUILD_LLVM_DYLIB:Bool=ON")
+
         # Get the GCC prefix for LLVM.
         if self.compiler.name == "gcc":
             compiler = Executable(self.compiler.cc)

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

@@ -170,6 +170,18 @@ class Llvm(CMakePackage, CudaPackage):
     variant('version_suffix', default='none', description="Add a symbol suffix")
     variant('z3', default=False, description='Use Z3 for the clang static analyzer')
 
+    provides('libllvm@13', when='@13.0.0:13')
+    provides('libllvm@12', when='@12.0.0:12')
+    provides('libllvm@11', when='@11.0.0:11')
+    provides('libllvm@10', when='@10.0.0:10')
+    provides('libllvm@9', when='@9.0.0:9')
+    provides('libllvm@8', when='@8.0.0:8')
+    provides('libllvm@7', when='@7.0.0:7')
+    provides('libllvm@6', when='@6.0.0:6')
+    provides('libllvm@5', when='@5.0.0:5')
+    provides('libllvm@4', when='@4.0.0:4')
+    provides('libllvm@3', when='@3.0.0:3')
+
     extends("python", when="+python")
 
     # Build dependency