AST: Don't walk all redeclarations when looking for a uuid attr

No funcionality changed, just a simplification of the existing code.
Give SplitBlockAndInsertIfThen the ability to update a domtree.
[dfsan] Move combineShadows to DFSanFunction in preparation for it to use a domtree.
[dfsan] Introduce an optimization to reduce the number of union queries.

Specifically, when building a union query, if we are dominated by an identical
query then use the result of that query instead.
Revert "[FastISel][X86] Remove no longer needed functions."
Revert "[FastISel][X86] Implement the FastLowerIntrinsicCall hook."
Revert "[FastISel][X86] Implement the FastLowerCall hook."

This reverts commit r213035, r213036, and r213037 to make the
buildbots happy again.