clang/Analysis: Fix r174245, a valgrind error in AnalysisDeclContext::getBody(bool &IsAutosynthesized), to initialize IsAutosynthesized explicitly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174303 91177308-0d34-0410-b5e6-96231b3b80d8