[libc++] Build <filesystem> support as part of the dylib

Summary:
This patch treats <filesystem> as a first-class citizen of the dylib,
like all other sub-libraries (e.g. <chrono>). As such, it also removes
all special handling for installing the filesystem library separately
or disabling part of the test suite from the lit command line.

Reviewers: mclow.lists, EricWF, serge-sans-paille

Subscribers: mgorny, christof, jkorous, dexonsmith, jfb, jdoerfert, libcxx-commits

Differential Revision: https://reviews.llvm.org/D59152

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@356500 91177308-0d34-0410-b5e6-96231b3b80d8
163 files changed