Preinstall some llvm shared libraries so tblgen will work. (#5988)

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

@@ -426,9 +426,16 @@ def cmake_args(self):
 
         if spec.satisfies('@4.0.0:') and spec.satisfies('platform=linux'):
             cmake_args.append('-DCMAKE_BUILD_WITH_INSTALL_RPATH=1')
-
         return cmake_args
 
+    @run_before('build')
+    def pre_install(self):
+        with working_dir(self.build_directory):
+            # When building shared libraries these need to be installed first
+            make('install-LLVMTableGen')
+            make('install-LLVMDemangle')
+            make('install-LLVMSupport')
+
     @run_after('install')
     def post_install(self):
         if '+clang' in self.spec and '+python' in self.spec: