[llvm][test][CGPluginTest] Keep plugin in shared library directory

Scoping to the root build directory instead of using the path directly is awkward and the only such
occurrence in the test suite. It's also prone to breakage for downstreams that change the library
path.

But it's not even necessary: during build we have the appropriate RPATHs set so we can just depend
on the dynamic loader to find it. This extra logic is probably just copy-paste from PluginsTest.cpp.

Additionally:
* Removed TargetParser as a dependency because it doesn't seem to actually be used.
* Moved `add_dependencies()` to `DEPENDS` to better match the rest of LLVM.

Author: tambry

llvm/test/Other/codegen-plugin-loading.ll

@@ -1,4 +1,4 @@
-; RUN: llc -load %llvmshlibdir/../unittests/CodeGen/CGPluginTest/CGTestPlugin%pluginext %s -o - | FileCheck %s
+; RUN: llc -load %llvm_obj_root/unittests/CodeGen/CGPluginTest/CGTestPlugin%pluginext %s -o - | FileCheck %s
 ; REQUIRES: native, system-linux, llvm-dylib
 
 ; CHECK: CodeGen Test Pass running on main

llvm/unittests/CodeGen/CGPluginTest/CMakeLists.txt

@@ -4,7 +4,6 @@ set(LLVM_LINK_COMPONENTS
   Core
   MC
   Target
-  TargetParser
   CodeGen
   )
 

llvm/unittests/CodeGen/CGPluginTest/Plugin/CMakeLists.txt

@@ -3,15 +3,12 @@ if (NOT WIN32 AND NOT CYGWIN)
   add_llvm_library(CGTestPlugin MODULE BUILDTREE_ONLY
     CodeGenTestPass.cpp
     Plugin.cpp
-    )
 
-  # Put PLUGIN next to the unit test executable.
-  set_output_directory(CGTestPlugin
-    BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}/../
-    LIBRARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}/../
+    DEPENDS
+    intrinsics_gen
+    vt_gen
     )
-  set_target_properties(CGTestPlugin PROPERTIES FOLDER "Tests")
 
-  add_dependencies(CGTestPlugin intrinsics_gen vt_gen)
+  set_target_properties(CGTestPlugin PROPERTIES FOLDER "Tests")
   add_dependencies(CGPluginTest CGTestPlugin)
 endif ()

llvm/unittests/CodeGen/CGPluginTest/PluginTest.cpp

@@ -25,20 +25,6 @@
 
 using namespace llvm;
 
-namespace {
-void anchor() {}
-
-std::string libPath(const std::string &Name) {
-  const auto &Argvs = testing::internal::GetArgvs();
-  const char *Argv0 = Argvs.size() > 0 ? Argvs[0].c_str() : "CGPluginTest";
-  void *Ptr = (void *)(intptr_t)anchor;
-  std::string Path = sys::fs::getMainExecutable(Argv0, Ptr);
-  SmallString<256> Buf{sys::path::parent_path(Path)};
-  sys::path::append(Buf, (Name + LLVM_PLUGIN_EXT).c_str());
-  return std::string(Buf.str());
-}
-} // namespace
-
 namespace llvm {
 class CGPluginTests : public testing::Test {
 protected:
@@ -56,8 +42,7 @@ TEST_F(CGPluginTests, LoadPlugin) {
   GTEST_SKIP();
 #endif
 
-  auto PluginPath = libPath("CGTestPlugin");
-  ASSERT_NE("", PluginPath);
+  auto PluginPath{std::string{"CGTestPlugin"} + LLVM_PLUGIN_EXT};
 
   std::string Error;
   auto Library = sys::DynamicLibrary::getLibrary(PluginPath.c_str(), &Error);