[NFC][TSan] Move libdispatch tests into their own subfolder

Remove 'gcd' file prefix. GCD stands for Grand Central Dispatch, which
is another name for libdispatch.
https://apple.github.io/swift-corelibs-libdispatch/

Remove `REQUIRE: dispatch` from tests.

Also rename lit feature 'dispatch' -> 'libdispatch' to be more explicit
what this is about.

Reviewed By: kubamracek

Differential Revision: https://reviews.llvm.org/D59341

llvm-svn: 356202