[ExpressionParser] Implement ComputeClangResourceDir for Windows

Summary: This function is useful for expression evaluation, especially when doing swift debugging on windows.

Reviewers: aprantl, labath

Reviewed By: labath

Subscribers: teemperor, jdoerfert, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D59072

llvm-svn: 355631