Make TableGenGlobalISel an object library

That's how it was originally intended but that wasn't possible because
we still needed to support older CMake versions.

The problem here is that the sources in TableGenGlobalISel are meant to
be linked into both llvm-tblgen and TableGenTests (a unit test), but not
be part of LLVM proper. So they shouldn't be an ordinary LLVM component.
Because they are used in llvm-tblgen, they can't draw in the LLVM dylib
dependency, but then we'd have to do the same thing in TableGenTests to
make sure we don't link both a static Support library and another copy
through the LLVM dylib.

With an object library we're just reusing the object files and don't
have to care about dependencies at all.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D74588

llvm/unittests/TableGen/CMakeLists.txt

@@ -12,6 +12,7 @@ add_public_tablegen_target(AutomataTestTableGen)
 add_llvm_unittest(TableGenTests
   CodeExpanderTest.cpp
   AutomataTest.cpp
+  $<TARGET_OBJECTS:obj.LLVMTableGenGlobalISel>
   )
 include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../../utils/TableGen)
-target_link_libraries(TableGenTests PRIVATE LLVMTableGenGlobalISel LLVMTableGen)
+target_link_libraries(TableGenTests PRIVATE LLVMTableGen)

llvm/utils/TableGen/CMakeLists.txt

@@ -56,6 +56,6 @@ add_tablegen(llvm-tblgen LLVM
   X86RecognizableInstr.cpp
   WebAssemblyDisassemblerEmitter.cpp
   CTagsEmitter.cpp
+  $<TARGET_OBJECTS:obj.LLVMTableGenGlobalISel>
   )
-target_link_libraries(llvm-tblgen PRIVATE LLVMTableGenGlobalISel)
 set_target_properties(llvm-tblgen PROPERTIES FOLDER "Tablegenning")

llvm/utils/TableGen/GlobalISel/CMakeLists.txt

@@ -3,7 +3,7 @@ set(LLVM_LINK_COMPONENTS
   TableGen
   )
 
-add_llvm_library(LLVMTableGenGlobalISel STATIC DISABLE_LLVM_LINK_LLVM_DYLIB
+add_llvm_library(LLVMTableGenGlobalISel OBJECT
                  CodeExpander.cpp
                  GIMatchDag.cpp
                  GIMatchDagEdge.cpp