[cmake] Revert part of r360991 "[Analysis] Only run plugins tests if plugins are actually enabled" that unilaterally disabled plugins

[hintonda]

Summary

This change makes is impossible to build plugins and test
them in the common case when doing a normal build and
BUILD_SHARED_LIBS and LLVM_BUILD_LLVM_DYLIB are both OFF, e.g., the
buildbots.

[phosek]

With this change, we're seeing failing tests in Clang:

error: 'error' diagnostics seen but not expected:
  (frontend): unable to load plugin './lib/SampleAnalyzerPlugin.so': './lib/SampleAnalyzerPlugin.so: undefined symbol: _ZN4llvm14FoldingSetBase6anchorEv'

I have a slightly larger patch that seems to address that and it also contains this one but I'm not sure if it's the right approach.

If we're going to land this change, then I'd argue for reverting D59464 as well.

[hintonda]

D62174 is a better solution.

llvm/cmake/modules/HandleLLVMOptions.cmake

if(BUILD_SHARED_LIBS AND LLVM_EXPORT_SYMBOLS_FOR_PLUGINS)
   message(FATAL_ERROR "BUILD_SHARED_LIBS not compatible with LLVM_EXPORT_SYMBOLS_FOR_PLUGINS")
 endif()
 if(LLVM_LINK_LLVM_DYLIB AND LLVM_EXPORT_SYMBOLS_FOR_PLUGINS)
   message(FATAL_ERROR "LLVM_LINK_LLVM_DYLIB not compatible with LLVM_EXPORT_SYMBOLS_FOR_PLUGINS")
 endif()
 
 # Plugin support
 # FIXME: Make this configurable.
+if(WIN32 OR CYGWIN)
   if(BUILD_SHARED_LIBS OR LLVM_BUILD_LLVM_DYLIB)
     set(LLVM_ENABLE_PLUGINS ON)
   else()
     set(LLVM_ENABLE_PLUGINS OFF)
   endif()
+else()
+  set(LLVM_ENABLE_PLUGINS ON)
+endif()
 
 # By default we should enable LLVM_ENABLE_IDE only for multi-configuration
 # generators. This option disables optional build system features that make IDEs
 # less usable.
 set(LLVM_ENABLE_IDE_default OFF)
 if (CMAKE_CONFIGURATION_TYPES)
   set(LLVM_ENABLE_IDE_default ON)
 endif()