[patch] Check the linkage cache at every recursive step

[Rafael Espíndola]

Currently the linkage cache is only used by the entry level function
(getLinkage). The function that does the actual computation
(getLVForDecl), never looks at it.

This means that we fail to reuse an entry in the cache when
getLVForDecl does a recursive call. The attached patch fixes that by
adding another computation enum value for when we don't care about the
linkage at all and having getLVForDecl check the cache in that case.

When running "clang -cc1" over SemaExpr.ii this brings the number of
linkage computations from 93749 to 58426. When running "clang -cc1
-emit-llvm -O3" it goes from 198708 to 161444.

Cheers,
Rafael
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