[CMake] Allow specifying extra dependencies of bootstrap stage

[phosek]

Summary

This allows adding additional bootstrap dependencies to the bootstrap
compiler that may be needed by later stages.

[phosek]

clang/CMakeLists.txt

586

I don't know if COMPONENTS is the best term, this could also possibly be DEPS.

[beanz]

I prefer DEPS to COMPONENTS because I've tried to explicitly use the term components to mean targets that have paired build and install targets. Otherwise looks good.

clang/CMakeLists.txt

elseif(NOT WIN32)
       if(NOT BOOTSTRAP_LLVM_ENABLE_LLD AND LLVM_BINUTILS_INCDIR)
         add_dependencies(clang-bootstrap-deps LLVMgold)
       endif()
       set(LTO_AR -DCMAKE_AR=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-ar)
       set(LTO_RANLIB -DCMAKE_RANLIB=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-ranlib)
     endif()
   endif()
 
+  if(CLANG_BOOTSTRAP_EXTRA_DEPS)
+    add_dependencies(clang-bootstrap-deps ${CLANG_BOOTSTRAP_EXTRA_DEPS})
+  endif()
+
   add_custom_target(${NEXT_CLANG_STAGE}-clear
     DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${NEXT_CLANG_STAGE}-cleared
     )
   add_custom_command(
     OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${NEXT_CLANG_STAGE}-cleared
     DEPENDS clang-bootstrap-deps
     COMMAND ${CMAKE_COMMAND} -E remove_directory ${BINARY_DIR}
     COMMAND ${CMAKE_COMMAND} -E make_directory ${BINARY_DIR}