[cmake] Create exports for umbrella library targets

[smeenai]

Summary

When using the umbrella llvm-libraries and clang-libraries targets, we
should export all library targets, otherwise they'll be part of our
distribution but not usable from the CMake package.

[phosek]

LGTM

clang/cmake/modules/AddClang.cmake

macro(add_clang_library name)
   llvm_add_library(${name} ${ARG_ENABLE_SHARED} ${ARG_UNPARSED_ARGUMENTS} ${srcs})
 
   if(TARGET ${name})
     target_link_libraries(${name} INTERFACE ${LLVM_COMMON_LIBS})
 
     if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY OR ${name} STREQUAL "libclang")
 
       if(${name} IN_LIST LLVM_DISTRIBUTION_COMPONENTS OR
+          "clang-libraries" IN_LIST LLVM_DISTRIBUTION_COMPONENTS OR
           NOT LLVM_DISTRIBUTION_COMPONENTS)
         set(export_to_clangtargets EXPORT ClangTargets)
         set_property(GLOBAL PROPERTY CLANG_HAS_EXPORTS True)
       endif()
 
       install(TARGETS ${name}
         COMPONENT ${name}
         ${export_to_clangtargets}

llvm/cmake/modules/AddLLVM.cmake

else()
     llvm_add_library(${name} ${ARG_UNPARSED_ARGUMENTS})
   endif()
 
   # Libraries that are meant to only be exposed via the build tree only are
   # never installed and are only exported as a target in the special build tree
   # config file.
   if (NOT ARG_BUILDTREE_ONLY AND NOT ARG_MODULE)
     set_property( GLOBAL APPEND PROPERTY LLVM_LIBS ${name} )
+    set(in_llvm_libs YES)
   endif()
 
   if (ARG_MODULE AND NOT TARGET ${name})
     # Add empty "phony" target
     add_custom_target(${name})
   elseif( EXCLUDE_FROM_ALL )
     set_target_properties( ${name} PROPERTIES EXCLUDE_FROM_ALL ON)
   elseif(ARG_BUILDTREE_ONLY)
if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY OR ${name} STREQUAL "LTO" OR
         set(install_type ARCHIVE)
       endif()
 
       if (ARG_MODULE)
         set(install_type LIBRARY)
       endif()
 
       if(${name} IN_LIST LLVM_DISTRIBUTION_COMPONENTS OR
+          (in_llvm_libs AND "llvm-libraries" IN_LIST LLVM_DISTRIBUTION_COMPONENTS) OR
           NOT LLVM_DISTRIBUTION_COMPONENTS)
         set(export_to_llvmexports EXPORT LLVMExports)
         set_property(GLOBAL PROPERTY LLVM_HAS_EXPORTS True)
       endif()
 
       install(TARGETS ${name}
               ${export_to_llvmexports}
               ${install_type} DESTINATION ${install_dir}