Reapply: [VFS] Add support for handling path traversals

This is originally r261551, reverted because of windows bots failing on
unittests. Change the current behavior to do not handle path traversals
on windows.

Handle ".", ".." and "./" with trailing slashes while collecting files
to be dumped into the vfs overlay directory.

Include the support for symlinks into components. Given the path:

/install-dir/bin/../lib/clang/3.8.0/include/altivec.h, if "bin"
component is a symlink, it's not safe to use `path::remove_dots` here,
and `realpath` is used to get the right answer. Since `realpath`
is expensive, we only do it at collecting time (which only happens
during the crash reproducer) and cache the base directory for fast lookups.

Overall, this makes the input to the VFS YAML file to be canonicalized
to never contain traversal components.

Differential Revision: http://reviews.llvm.org/D17104

rdar://problem/24499339

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263617 91177308-0d34-0410-b5e6-96231b3b80d8