Use task local random when initializing LinearMap

Add test for float::round

librustdoc: De-export compiletest, combine-tests, libcargo, libfuzzer, and librustdoc. rs=deexporting

Remove CFG_LLVM_CONFIG probe from configure script

This is probably to work around a bug.

Add a new method_super origin for supertrait methods. Also make
coherence create a table that maps pairs of trait IDs and self types
to impl IDs, so that it's possible to check a supertrait method
knowing only its index in its trait's methods (without knowing all
supertraits for a given trait).

r=nmatsakis and graydon -- with hope, we'll revamp all of this code as
per #4678, but for now this fixes the bug.

Closes #3979

Set trait improvements + minor treemap cleanup