lld::COFF: better behavior when using as a library

Previously, the COFF driver would call exit(0) when called
as a library.  Now it takes `ExitEarly` option, and if it
is false, it doesn't exit.  So it is now more library-friendly.

Furthermore, link() calls freeArena() before returning, to
clean up resources.

Based on an Andrew Kelley's patch.

Differential Revision: https://reviews.llvm.org/D39202

llvm-svn: 316370