The embedded shell script in the RUN command for lit is problematic for
non-sh shell environments (i.e. Windows). This adjusts the tests to
uniformly build the code for the ObjC runtime. However, the Objective-C
code is only built under the same circumstances that it is currently
enabled - the availability of the needed frameworks. The empty object
on other runtimes will have no material impact. The swift side of it
checks whether the runtime is built with ObjC interop. This allows us
to largely use the same command line for all the targets. The last
missing piece is that the `-fobjc-runtime` requires that we run a modern
ObjC runtime. We enable this unconditionally in lit for the non-Apple
targets.
This improves the validation test coverage for the standard library on
Windows.
After rebasing on master and incorporating more 32-bit support,
perform a bunch of cleanup, documentation updates, comments, move code
back to String declaration, etc.
