[patch] Check the linkage cache at every recursive step

[Rafael Espíndola]

> Well, wait, associated question — does this actually cause a speed-up?

For SemaExpr.ii it is in the noise, which is why I decided to count
the number of linkage computations.

The patch has some other advantages:
* verifyLinkage is gone (inlined to getLVForDecl)
* When asserts are enabled, we check the linkage even when it is
computed as part of a recursive getLVForDecl. That is how I found the
bug fixed in r182773.

OK?

Cheers,
Rafael