Add function DominatorTree::getDescendants().

  As its name suggests, this function will return all basic blocks
dominated by a given block.
msbuild: Set _MSC_VER to match the CRT we're using

Various Windows SDK headers use _MSC_VER values to figure out what
version of the VC++ headers they're using, in particular for SAL macros.

Patch by Paul Hampson!
Refinement to my previous patch for
objc_returns_inner_pointer on properties. // rdar://14990439