[llvm-jitlink] Update handling of library options.

Adds -L<search-path> and -l<library> options that are analogous to ld's
versions.

Each instance of -L<search-path> or -l<library> will apply to the most recent
-jd option on the command line (-jd <name> creates a JITDylib with the given
name). Library names will match against JITDylibs first, then llvm-jitlink will
look through the search paths for files named <search-path>/lib<library>.dylib
or <search-path>/lib<library>.a.

The default "main" JITDylib will link against all JITDylibs created by -jd
options, and all JITDylibs will link against the process symbols (unless
-no-process-symbols is specified).

The -dlopen option is renamed -preload, and will load dylibs into the JITDylib
for the ORC runtime only.

The effect of these changes is to make it easier to describe a non-trivial
program layout to llvm-jitlink for testing purposes. E.g. the following
invocation describes a program consisting of three JITDylibs: "main" (created
implicitly) containing main.o, "Foo" containing foo1.o and foo2.o, and linking
against library "bar" (not a JITDylib, so it must be a .dylib or .a on disk)
and "Baz" (which is a JITDylib), and "Baz" containing baz.o.

llvm-jitlink \
  main.o \
  -jd Foo foo1.o foo2.o -L${HOME}/lib -lbar -lBaz
  -jd Baz baz.o