Fix use of add_llvm_tool in projects that build no targets

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.

https://reviews.llvm.org/D70167

Signed-off-by: Kevin Petit <kevin.petit@arm.com>

llvm/cmake/modules/LLVM-Config.cmake

@@ -28,7 +28,7 @@ function(is_llvm_target_library library return_var)
     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)