CMake: Fix stand-alone clang builds since r353268

Summary:
Handle the case where LLVM_MAIN_SRC_DIR is not set and also use
LLVM_CMAKE_DIR for locating installed cmake files rather than
LLVM_CMAKE_PATH.

Reviewers: phosek, andrewrk, smeenai

Reviewed By: phosek, andrewrk, smeenai

Subscribers: mgorny, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D58204

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354417 91177308-0d34-0410-b5e6-96231b3b80d8

Author: tstellar

cmake/modules/AddLLVM.cmake

@@ -1718,6 +1718,9 @@ function(setup_dependency_debugging name)
 endfunction()
 
 function(find_first_existing_vc_file path out_var)
+  if(NOT EXISTS "${path}")
+    return()
+  endif()
   if(EXISTS "${path}/.svn")
     set(svn_files
       "${path}/.svn/wc.db"   # SVN 1.7