[llvm][cmake] Improve error message about unknown build targets

[tbaeder]

Summary

If a build target was previously not in LLVM_ALL_TARGETS, but also not
in LLVM_EXPERIMENTAL_TARGETS_TO_BUILD, cmake would still tell people to
pass it via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.

[beanz]

The current workflow for experimental targets is something like:

cmake <args> -DLLVM_TARGETS_TO_BUILD=<targets llvm knows about> -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=MyTarget

If I'm reading your cage correctly, you make this an error, instead requiring:

cmake <args> -DLLVM_TARGETS_TO_BUILD=<targets llvm knows about>;MyTarget -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=MyTarget

I'm not sure that's the workflow change we want.

llvm/CMakeLists.txt

foreach(t ${LLVM_TARGETS_TO_BUILD})
   set( td ${LLVM_MAIN_SRC_DIR}/lib/Target/${t} )
 
   list(FIND LLVM_ALL_TARGETS ${t} idx)
   list(FIND LLVM_EXPERIMENTAL_TARGETS_TO_BUILD ${t} idy)
   # At this point, LLVMBUILDTOOL already checked all the targets passed in
   # LLVM_TARGETS_TO_BUILD and LLVM_EXPERIMENTAL_TARGETS_TO_BUILD, so
   # this test just makes sure that any experimental targets were passed via
   # LLVM_EXPERIMENTAL_TARGETS_TO_BUILD, not LLVM_TARGETS_TO_BUILD.
-  if( idx LESS 0 AND idy LESS 0 )
+  if( idx LESS 0 AND idy GREATER_EQUAL 0 )
     message(FATAL_ERROR "The target `${t}' is experimental and must be passed "
       "via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.")
+  elseif( idx LESS 0 AND idy LESS 0 )
+    message(FATAL_ERROR "The target `${t}' is neither a supported target nor "
+      "an experimental target")
   else()
     set(LLVM_ENUM_TARGETS "${LLVM_ENUM_TARGETS}LLVM_TARGET(${t})\n")
   endif()
 
   file(GLOB asmp_file "${td}/*AsmPrinter.cpp")
   if( asmp_file )
     set(LLVM_ENUM_ASM_PRINTERS
       "${LLVM_ENUM_ASM_PRINTERS}LLVM_ASM_PRINTER(${t})\n")