Add typed nodes to SemIR. (#3280)

Replace `SemIR::Node::GetAsFoo` and `SemIR::Node::Foo::Make` with
`SemIR::Foo` class that represents a particular kind of node, with named
fields.

Rename `SemIR::IntegerLiteral` and `SemIR::RealLiteral` to
`IntegerValue` / `RealValue` to better reflect their purpose and avoid a
name collision with the corresponding `SemIR` node kinds.

Remove `NodeKind::Invalid` and the `SemIR::Node` default constructor
entirely, as they were not used for anything.

toolchain/lower/function_context.cpp

@@ -44,9 +44,9 @@ auto FunctionContext::LowerBlock(SemIR::NodeBlockId block_id) -> void {
     // clang warns on unhandled enum values; clang-tidy is incorrect here.
     // NOLINTNEXTLINE(bugprone-switch-missing-default-case)
     switch (node.kind()) {
-#define CARBON_SEMANTICS_NODE_KIND(Name) \
-  case SemIR::NodeKind::Name:            \
-    Handle##Name(*this, node_id, node);  \
+#define CARBON_SEMANTICS_NODE_KIND(Name)                  \
+  case SemIR::NodeKind::Name:                             \
+    Handle##Name(*this, node_id, node.As<SemIR::Name>()); \
     break;
 #include "toolchain/sem_ir/node_kind.def"
     }