Fix check-lit compatibility with multi-config CMake generators

Multi-configuration CMake generators such as those for Visual Studio or Xcode do not
specify a build config at configure time, but let the user choose at build
time.  In these cases binaries go into build/${Configuration}/bin rather than
build/bin.  Prior to this commit, check-lit would fail when using multi-configuration
generators as it did not know how to resolve ${Configuration} in order
to find tools such as FileCheck.  This commit teaches it to resolve
llvm_tools_dir within lit using the value specified with --param
build_mode.

Differential Revision: https://reviews.llvm.org/D36263

llvm-svn: 309967