Handle Windows drive letters and ".." for --reproduce.

When --reproduce <path> is given, then we need to concatenate input
file paths to the given path to save input files to the directory.
Previously, path concatenation didn't handle Windows drive letters
so it could generate invalid paths such as "C:\D:\foo". It also didn't
handle ".." path components, so it could produce some bad paths
such as "foo/../../etc/passwd".

In this patch, Windows drive letters and ".." are removed before
concatenating paths.

Differential Revision: http://reviews.llvm.org/D19551

llvm-svn: 267600