[Support] Add sys::fs::set_current_path() (aka chdir)

[Support] Add sys::fs::set_current_path() (aka chdir)

Summary:
This adds a cross-platform way of setting the current working directory
analogous to the existing current_path() function used for retrieving
it. The function will be used in lldb.

Reviewers: rafael, silvas, zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29035

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292907 91177308-0d34-0410-b5e6-96231b3b80d8