unittests/CMakeLists.txt: Suppress building ValueMapTest on MSVC older than 10(VS2010).

MSVC9 and 8 cannot compile ValueMapTest.cpp due to their bug.
See issue#331418 in Visual Studio.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119782 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed