Centralize libc++ test skipping logic

Summary:
This aims to replace the different decorators we've had on each libc++
test with a single solution. Each libc++ will be assigned to the
"libc++" category and a single central piece of code will decide whether
we are actually able to run libc++ test in the given configuration by
enabling or disabling the category (while giving the user the
opportunity to override this).

I started this effort because I wanted to get libc++ tests running on
android, and none of the existing decorators worked for this use case:
 - skipIfGcc - incorrect, we can build libc++ executables on android
 with gcc (in fact, after this, we can now do it on linux as well)
 - lldbutil.skip_if_library_missing - this checks whether libc++.so is
 loaded in the proces, which fails in case of a statically linked
 libc++ (this makes copying executables to the remote target easier to
 manage).

To make this work I needed to split out the pseudo_barrier code from the
force-included file, as libc++'s atomic does not play well with gcc on
linux, and this made every test fail, even though we need the code only
in the threading tests.

So far, I am only annotating one of the tests with this category. If
this does not break anything, I'll proceed to update the rest.

Reviewers: jingham, zturner, EricWF

Subscribers: srhines, lldb-commits

Differential Revision: https://reviews.llvm.org/D30984

llvm-svn: 299028

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