Renamed "CFG::BuildCFG" to "CFG::buildCFG" to have more consistent capitalization.

Added explicit "Exit" CFGBlock pointer to the source-level CFG.

Changed the construction of blocks with "return" statements to have the
return statement appear both as a statement in the list of statements for
a CFGBlock as well as appear as a control-flow terminator.  Also removed
the implicit linearization of "return" so that the return value and the
return statement did not appear as separate statements in the block.

llvm-svn: 41323