llvm-dwp: Improve performance (N^2 to amortized N) by using a MapVector...

llvm-dwp: Improve performance (N^2 to amortized N) by using a MapVector instead of linear searches through a vector

Figured this would be a problem, but didn't want to jump the gun - large
inputs demonstrate it pretty easily (mostly for type units, but might as
well do the same for CUs too). A random sample 6m27s -> 27s change.

Also, by checking this up-front for CUs (rather than when building the
cu_index) we can probably provide better error messages (see FIXMEs),
hopefully providing the name of the CUs rather than just their
signature.

llvm-svn: 261364