FileSpec: Fix PrependPathComponent("/")

Summary:
PrependPathComponent was unconditionally inserting path separators between the
path components. This is not correct if the prepended path is "/", which caused
problems down the line. Fix the function to use the same algorithm as
AppendPathComponent and add a test. This fixes one part of llvm.org/pr31611.

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D28677

llvm-svn: 292100