Remove C++ default arg side table for MS ABI ctor closures
We don't need a side table in ASTContext to hold CXXDefaultArgExprs. The
important part of building the CXXDefaultArgExprs was to ODR use the
default argument expressions, not to make AST nodes. Refactor the code
to only check the default argument, and remove the side table in
ASTContext which wasn't being serialized.
Reviewers: thakis, rsmith, majnemer
Differential Revision: https://reviews.llvm.org/D27007