Make explicit the single-argument constructors of AttributeCommonInfo; NFC

The single-argument constructors of this class were not marked explicit
and that led to some incorrect uses that slipped under the radar (see
changes in SemaDeclAttr.cpp). This makes the constructors explicit,
changes the benignly incorrect uses, and updates the tablegen code to
emit the correct support code to call the explicit constructors.

While this does correct a misuse, that incorrect usage could not be
observed except via a debugger and so no additional tests are added.

Differential Revision: https://reviews.llvm.org/D147661