[clang] Make `InitListExpr::isExplicit()` work (#195175)
The main goal of this change is to be able to use `isExplicit()` check
in the IWYU tool. Consider the following:
```cpp
struct Inner {};
struct Outer {
Inner inner;
};
const Outer& refOuter = {};
```
Here, Clang generates `InitListExpr` child node for the implicit `Inner`
initialization under the `InitListExpr` node corresponding to the
initializer of `refOuter`. IWYU should require the header containing
`Outer` definition for the initializer, but not the header for `Inner`
because it should be already provided by `Outer`.
'IsExplicit' flag is copied from a template instantiation pattern into
its instantiations although they are implicit because it makes sense for
IWYU at least. (After all, instantiated declarations refer to the
pattern explicitly written in the source.)
`isImplicit()` AST matcher has been extended to support `InitListExpr`s.
This is an NFC from the point of view of users of the Clang standalone
executable (except the change in AST dumping), but a functional change
for those who use Clang as a library.
