Sink add_llvm_library(gtest_main) to UnitTestMain/CMakeLists.txt.

llvm-svn: 198933
diff --git a/llvm/utils/unittest/CMakeLists.txt b/llvm/utils/unittest/CMakeLists.txt
index fd1a048..569af88 100644
--- a/llvm/utils/unittest/CMakeLists.txt
+++ b/llvm/utils/unittest/CMakeLists.txt
@@ -42,6 +42,4 @@
   googletest/src/gtest-all.cc
   )
 
-add_llvm_library(gtest_main
-  UnitTestMain/TestMain.cpp
-  )
+add_subdirectory(UnitTestMain)