Change CommandObjectTargetModulesLoad so that the filename argument
is treated as a string instead of a FileSpec.

OptionValueFileSpec::SetValueFromCString() passes the c string to
FileSpec::SetFile(str, true /* resolve */) - and with Zachary's
changes to FileSpec we're using llvm::sys::fs::make_absolute() to
do that "resolve" action now, where we used to use realpath().

One important difference between llvm::sys::fs::make_absolute and
realpath is that when they're handed a filename (no directory),
realpath prepends the current working directory *and if the file exists*,
returns that full path.  If that file doesn't exist, the caller
uses the basename only.

llvm::sys::fs::make_absolute prepends the current working directory
regardless of whether it exists or not.

I considered having FileSpec::SetFile save the initial pathname,
call FileSpec::Resolve, and then check to see if the Resolve return
path exists - and if not, go back to the original one.

But instead I just went with changing 'target modules load' to treat its
filename argument as a string instead of a FileSpec.  This brings it
in line with how 'target modules list' works.

<rdar://problem/18955416>