Provide better error messages for incorrect matchers.

By changing the conversion operator into a conversion constructor, we
can enabled based on the template parameters leading to better error
messages. E.g.: stmt(decl()) will now create an error message including:

  note: candidate function not viable: no known conversion from
  'clang::ast_matchers::internal::BindableMatcher<clang::Decl>' to 'const
  clang::ast_matchers::internal::Matcher<clang::Stmt>' for 1st argument

llvm-svn: 164298