Don't try to guess svn version when current path is a symlink.

See PR 8473.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@117110 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Basic/CMakeLists.txt b/lib/Basic/CMakeLists.txt
index 87bf834..3f737e7 100644
--- a/lib/Basic/CMakeLists.txt
+++ b/lib/Basic/CMakeLists.txt
@@ -17,7 +17,9 @@
 # Determine Subversion revision.
 # FIXME: This only gets updated when CMake is run, so this revision number
 # may be out-of-date!
-find_package(Subversion)
+if( NOT IS_SYMLINK "${CLANG_SOURCE_DIR}" )  # See PR 8437
+  find_package(Subversion)
+endif()
 if (Subversion_FOUND AND EXISTS "${CLANG_SOURCE_DIR}/.svn")
   Subversion_WC_INFO(${CLANG_SOURCE_DIR} CLANG)
   set_source_files_properties(Version.cpp