PR11926 + duplicates: Fix crash in -Wuninitialized when using a compiler like

g++4.7, which reuses stack space allocated for temporaries. CFGElement::getAs
returns a suitably-cast version of 'this'. Patch by Markus Trippelsdorf!

No test: this code has the same observable behavior as the old code when built
with most compilers, and the tests were already failing when built with a
compiler for which this produced a broken binary.

llvm-svn: 155803