Use KEY_WOW64_32KEY when reading the registry in WindowsToolChain.cpp (PR17033)

This exposes a 32-bit view of the registry even when Clang is built as a 64-bit
program. Since Visual Studio is a 32-bit application, this is necessary for us
to find it.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@192331 91177308-0d34-0410-b5e6-96231b3b80d8