[llvm-config] Fix obviously wrong code in parsing DyLib components.

The code parsing the string was using the offset returned from
StringRef::find() wrong, assuming it was relative to the staring
offset that is passed to the function, but the returned offset
is always relative to the beginning of the line.

This causes odd behaviour while parsing the component string.
Spotted thanks to the newly added test:

tools/llvm-config/booleans.test

llvm-svn: 291803