Fix Objective-C compilation-time performance regression introduced in r152608.

Reintroduce lazy name lookup table building, ensuring that the lazy building step
produces the same lookup table that would be built by the eager step.

Avoid building a lookup table for the translation unit outside C++, even in cases
where we can't recover the contents of the table from the declaration chain on
the translation unit, since we're not going to perform qualified lookup into it
anyway. Continue to support lazily building such lookup tables for now, though,
since ASTMerge uses them.

In my tests, this performs very similarly to ToT with r152608 backed out, for C,
Obj-C and C++, and does not suffer from PR10447.

llvm-svn: 152905