Added a new API, swift::driver::createCompilerInvocation. This takes an array
of driver arguments, constructs a Driver and a Compilation, and then uses the
Compilation's frontend arguments to create a CompilerInvocation.
This works by forcing Driver to create a Compilation which contains a single
compile command. (It achieves this by passing
"-force-single-frontend-invocation" after all other arguments.) This approach
roughly matches Clang's clang::createInvocationFromCommandLine.
As implied by the namespacing, this lives in swiftDriver. As a result,
swiftDriver now depends on swiftFrontend.
In support of this, added a couple of driver diagnostics for exceptional error
cases (where Driver produced something other than a single Command, or if that
Command is not a frontend command).
This fixes <rdar://problem/16125395>.
Swift SVN r20972
