Thread safety analysis: Handle additional cast in scoped capability construction

We might have a CK_NoOp cast and a further CK_ConstructorConversion.
As an optimization, drop some IgnoreParens calls: inside of the
CK_{Constructor,UserDefined}Conversion should be no more parentheses,
and inside the CXXBindTemporaryExpr should also be none.

Lastly, we factor out the unpacking so that we can reuse it for
MaterializeTemporaryExprs later on.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D129752