Merging r155803:

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r155803 | rsmith | 2012-04-29 17:16:51 -0700 (Sun, 29 Apr 2012) | 8 lines

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.

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llvm-svn: 156394