[ASTMatchers] Fix build when no targets are enabled

[steveire]

Summary

This makes sense to do when building only tools like clang-tidy for
example.

[aaron.ballman]

llvm/cmake/modules/LLVM-Config.cmake

31–33

I'm not a CMake expert, so can you explain what effect this change has? From the documentation (and my own limited understanding of cmake), I would have expected this to be a noop -- if the list of elements is empty, nothing will get removed from omitted_targets, right?

[steveire]

llvm/cmake/modules/LLVM-Config.cmake

31–33

You can reproduce the issue in a small testcase with this file:

set(omitted_targets one two three)

# Comment this to reproduce the failure
set(LLVM_TARGETS_TO_BUILD X86)

list(REMOVE_ITEM omitted_targets ${LLVM_TARGETS_TO_BUILD})

Run this after commenting the line:

$ cmake -P cmaketest.cmake
CMake Error at cmaketest.cmake:6 (list):
  list sub-command REMOVE_ITEM requires two or more arguments.

[aaron.ballman]

LGTM

llvm/cmake/modules/LLVM-Config.cmake

31–33

Huh, wow, how unintuitive! Thank you for the explanation.

[steveire]

llvm/cmake/modules/LLVM-Config.cmake

31–33

Yes, I've filed https://gitlab.kitware.com/cmake/cmake/-/issues/21663

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})
+    if (LLVM_TARGETS_TO_BUILD)
       list(REMOVE_ITEM omitted_targets ${LLVM_TARGETS_TO_BUILD})
+    endif()
     string(TOUPPER "${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