CMake: Disable ENABLE_EXPORTS for executables with MSVC

The MSVC linker won't produce a .lib file for an executable that doesn't
export anything, and LLVM doesn't maintain dllexport annotations or .def
files listing all C++ symbols. It also doesn't support exporting all
symbols, like binutils ld.

CMake 3.2 changed the Ninja generator to list both the .exe and .lib
files as outputs of executable build targets. Ninja would always re-link
executables with ENABLE_EXPORTS because the .lib output file was not
present, and therefore the target was out of date.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232662 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/modules/AddLLVM.cmake

@@ -490,6 +490,12 @@ macro(add_llvm_executable name)
   endif( LLVM_COMMON_DEPENDS )
 endmacro(add_llvm_executable name)
 
+function(export_executable_symbols target)
+  if (NOT MSVC) # MSVC's linker doesn't support exporting all symbols.
+    set_target_properties(${target} PROPERTIES ENABLE_EXPORTS 1)
+  endif()
+endfunction()
+
 
 set (LLVM_TOOLCHAIN_TOOLS
   llvm-ar