[CMake] More cleanup of installing symlinks.

In order to support building clang out-of-tree the install_symlink script needs to be installed, and it needs to be found by searching the CMAKE_MODULE_PATH.

This change renames install_symlink -> LLVMInstallSymlink so it doesn't conflict with naming from other projects, and adds searching behavior in AddLLVM.cmake

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248009 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/modules/AddLLVM.cmake

@@ -1024,7 +1024,13 @@ function(add_lit_testsuites project directory)
 endfunction()
 
 function(llvm_install_symlink name dest)
-  install(SCRIPT ${CMAKE_SOURCE_DIR}/cmake/modules/install_symlink.cmake
+  foreach(path ${CMAKE_MODULE_PATH})
+    if(EXISTS ${path}/LLVMInstallSymlink.cmake)
+      set(INSTALL_SYMLINK ${path}/LLVMInstallSymlink.cmake)
+      break()
+    endif()
+  endforeach()
+  install(SCRIPT ${INSTALL_SYMLINK}
           CODE "install_symlink(${name} ${dest})"
           COMPONENT ${name})