[analyzer] Support C++ default arguments if they are literal values.

A CXXDefaultArgExpr wraps an Expr owned by a ParmVarDecl belonging to the
called function. In general, ExprEngine and Environment ought to treat this
like a ParenExpr or other transparent wrapper expression, with the inside
expression evaluated first.

However, if we call the same function twice, we'd produce a CFG that contains
the same wrapped expression twice, and we're not set up to handle that. I've
added a FIXME to the CFG builder to come back to that, but meanwhile we can
at least handle expressions that don't need to be explicitly evaluated:
literals. This probably handles many common uses of default parameters:
true/false, null, etc.

Part of PR13385 / <rdar://problem/12156507>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162453 91177308-0d34-0410-b5e6-96231b3b80d8

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