[RecursiveASTVisitor] Visit the implicit expression of a CXXDefaultArgExpr

Summary:
The matcher
varDecl(hasDescendant(

callExpr(hasDeclaration(functionDecl(unless(isNoThrow()))))))

didn't match calls from default arguments because the expression
for a CXXDefaultArgExpr was not visited.

Reviewers: klimek, jdennett, alexfh, aaron.ballman

Subscribers: aaron.ballman, cfe-commits

Differential Revision: https://reviews.llvm.org/D25992