This reverts commit e476f968bc8e438a0435d10934f148de570db8eb.
It has introduced a failure tracked by https://github.com/llvm/llvm-project/issues/150601
One libcxx test fail if libcxx is build with no exceptions and no RTTI:
- libcxx/utilities/expected/expected.expected/value.observers.verify.cpp
Added explanation why a is constructible evaluated to false. Also fixed
problem with ExtractTypeTraitFromExpression. In case std::is_xxx_v<>
with variadic pack it tries to get template argument, but fails in
expression Arg.getAsType() due to Arg.getKind() ==
TemplateArgument::ArgKind::Pack, but not
TemplateArgument::ArgKind::Type.
Reverts #144127
Fixies
https://github.com/llvm/llvm-project/pull/143309#issuecomment-2970012054
