Make all VariadicDynCast-Expr-matchers StatementMatchers

Having Expr-subclass-matchers being VariadicDynCast-Expr-matchers does
not provide additional type safety and just makes using them harder. In
the clang AST, expressions are always statements and an expression
matcher can be used anywhere, a statement matcher is expected. Any given
matcher definition still has at least one noun.

llvm-svn: 164121