cmake: Don't install plugins used for examples or tests

[tstellar]

Summary

This patch drops install targets for LLVMHello.so,
TestPlugin.so, and BugpointPasses.so.

[SquallATF]

with this patch, cmake still failed when build LLVMgold in mingw

CMake Error at cmake/modules/AddLLVM.cmake:667 (install):
  install TARGETS given no LIBRARY DESTINATION for module target "LLVMgold".
Call Stack (most recent call first):
  tools/gold/CMakeLists.txt:14 (add_llvm_library)

[SquallATF]

Module libraries are always treated as LIBRARY targets. via https://cmake.org/cmake/help/v3.0/command/install.html, but D51748 marked as RUNTIME targets when build with mingw

[tstellar]

In D55965#1338809, @SquallATF wrote:

Module libraries are always treated as LIBRARY targets. via https://cmake.org/cmake/help/v3.0/command/install.html, but D51748 marked as RUNTIME targets when build with mingw

Does D56004 fix the build failures?

[chandlerc]

FWIW, this LGTM. Clearly make sure that any build issues are resolved, but the core idea makes sense to me so land it whenever ready.

llvm/trunk/lib/Transforms/Hello/CMakeLists.txt

 # If we don't need RTTI or EH, there's no reason to export anything
 # from the hello plugin.
 if( NOT LLVM_REQUIRES_RTTI )
   if( NOT LLVM_REQUIRES_EH )
     set(LLVM_EXPORTED_SYMBOL_FILE ${CMAKE_CURRENT_SOURCE_DIR}/Hello.exports)
   endif()
 endif()
 
 if(WIN32 OR CYGWIN)
   set(LLVM_LINK_COMPONENTS Core Support)
 endif()
 
-add_llvm_library( LLVMHello MODULE
+add_llvm_library( LLVMHello MODULE BUILDTREE_ONLY
   Hello.cpp
 
   DEPENDS
   intrinsics_gen
   PLUGIN_TOOL
   opt
   )

llvm/trunk/tools/bugpoint-passes/CMakeLists.txt

 if( NOT LLVM_BUILD_TOOLS )
   set(EXCLUDE_FROM_ALL ON)
 endif()
 
 # If we don't need RTTI or EH, there's no reason to export anything
 # from this plugin.
 if( NOT LLVM_REQUIRES_RTTI )
   if( NOT LLVM_REQUIRES_EH )
     set(LLVM_EXPORTED_SYMBOL_FILE ${CMAKE_CURRENT_SOURCE_DIR}/bugpoint.exports)
   endif()
 endif()
 
 if(WIN32 OR CYGWIN)
   set(LLVM_LINK_COMPONENTS Core)
 endif()
 
-add_llvm_library( BugpointPasses MODULE
+add_llvm_library( BugpointPasses MODULE BUILDTREE_ONLY
   TestPasses.cpp
 
   DEPENDS
   intrinsics_gen
   bugpoint
   )

llvm/trunk/unittests/Passes/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS Support Passes Core)
 add_llvm_unittest(PluginsTests
   PluginsTest.cpp
   )
 export_executable_symbols(PluginsTests)
 target_link_libraries(PluginsTests PRIVATE LLVMTestingSupport)
 
 set(LLVM_LINK_COMPONENTS)
-add_llvm_library(TestPlugin MODULE
+add_llvm_library(TestPlugin MODULE BUILDTREE_ONLY
   TestPlugin.cpp
   )
 
 # Put plugin next to the unit test executable.
 set_output_directory(TestPlugin
   BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}
   LIBRARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}
   )
 set_target_properties(TestPlugin PROPERTIES FOLDER "Tests")
 
 add_dependencies(TestPlugin intrinsics_gen)
 add_dependencies(PluginsTests TestPlugin)