Install libLLVM if needed with LLVM_INSTALL_TOOLCHAIN_ONLY

[sbc100]

Summary

When LLVM_LINK_LLVM_DYLIB is set, the libLLVM shared
library needs to be installed in the toolchain. Without
this chanage LLVM_INSTALL_TOOLCHAIN_ONLY combined with
LLVM_LINK_LLVM_DYLIB results in a broken install.

This change will allow the workaround in the WASM
waterfall to be removed:
https://github.com/WebAssembly/waterfall/blob/master/src/build.py#L710

[sbc100]

My first llvm change... please excuse my lack of knowledge.

[beanz]

LGTM.

cmake/modules/AddLLVM.cmake

if (NOT ARG_BUILDTREE_ONLY)
     set_property( GLOBAL APPEND PROPERTY LLVM_LIBS ${name} )
   endif()
 
   if( EXCLUDE_FROM_ALL )
     set_target_properties( ${name} PROPERTIES EXCLUDE_FROM_ALL ON)
   elseif(ARG_BUILDTREE_ONLY)
     set_property(GLOBAL APPEND PROPERTY LLVM_EXPORTS_BUILDTREE_ONLY ${name})
   else()
-    if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY OR ${name} STREQUAL "LTO")
+    if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY OR ${name} STREQUAL "LTO" OR
+        (LLVM_LINK_LLVM_DYLIB AND ${name} STREQUAL "LLVM"))
       set(install_dir lib${LLVM_LIBDIR_SUFFIX})
       if(ARG_SHARED OR BUILD_SHARED_LIBS)
         if(WIN32 OR CYGWIN OR MINGW)
           set(install_type RUNTIME)
           set(install_dir bin)
         else()
           set(install_type LIBRARY)
         endif()