[CFG] [analyzer] pr41142: C++17: Skip transparent InitListExprs in constructors.

When searching for construction contexts, i.e. figuring out which statements
define the object that is constructed by each construct-expression, ignore
transparent init-list expressions because they don't add anything to the
context. This allows the Static Analyzer to model construction, destruction,
materialization, lifetime extension correctly in more cases. Also fixes
a crash caused by incorrectly evaluating initial values of variables
initialized with such expressions.

Differential Revision: https://reviews.llvm.org/D59573

llvm-svn: 356634