Fix use of add_llvm_tool in projects that build no targets

[kpet]

Summary

Projects that set LLVM_TARGETS_TO_BUILD to an empty list
can't use add_llvm_tool (and probably other macros).

Here's the error that this change fixes:

list sub-command REMOVE_ITEM requires two or more arguments.

Signed-off-by: Kevin Petit <[e-mail]>

[svenvh]

LGTM!

[kpet]

Committed as e73177ea5fd611026abcbaecc6232eee8d8d2ed8.

llvm/cmake/modules/LLVM-Config.cmake

function(is_llvm_target_library library return_var)
   # Sets variable `return_var' to ON if `library' corresponds to a
   # LLVM supported target. To OFF if it doesn't.
   set(${return_var} OFF PARENT_SCOPE)
   string(TOUPPER "${library}" capitalized_lib)
   if(ARG_INCLUDED_TARGETS)
     string(TOUPPER "${LLVM_TARGETS_TO_BUILD}" targets)
   elseif(ARG_OMITTED_TARGETS)
     set(omitted_targets ${LLVM_ALL_TARGETS})
-    list(REMOVE_ITEM omitted_targets ${LLVM_TARGETS_TO_BUILD})
+    list(REMOVE_ITEM omitted_targets "${LLVM_TARGETS_TO_BUILD}")
     string(TOUPPER "${omitted_targets}" targets)
   else()
     string(TOUPPER "${LLVM_ALL_TARGETS}" targets)
   endif()
   foreach(t ${targets})
     if( capitalized_lib STREQUAL t OR
         capitalized_lib STREQUAL "${t}" OR
         capitalized_lib STREQUAL "${t}DESC" OR