Introduce if/switch expressions

Introduce SingleValueStmtExpr, which allows the
embedding of a statement in an expression context.
This then allows us to parse and type-check `if`
and `switch` statements as expressions, gated
behind the `IfSwitchExpression` experimental
feature for now. In the future,
SingleValueStmtExpr could also be used for e.g
`do` expressions.

For now, only single expression branches are
supported for producing a value from an
`if`/`switch` expression, and each branch is
type-checked independently. A multi-statement
branch may only appear if it ends with a `throw`,
and it may not `break`, `continue`, or `return`.

The placement of `if`/`switch` expressions is also
currently limited by a syntactic use diagnostic.
Currently they're only allowed in bindings,
assignments, throws, and returns. But this could
be lifted in the future if desired.

lib/SILGen/SILGenFunction.cpp

@@ -1237,6 +1237,18 @@ void SILGenFunction::emitGeneratorFunction(
   mergeCleanupBlocks();
 }
 
+Initialization *SILGenFunction::getSingleValueStmtInit(Expr *E) {
+  if (SingleValueStmtInitStack.empty())
+    return nullptr;
+
+  // Check to see if this is an expression branch of an active
+  // SingleValueStmtExpr initialization.
+  if (!SingleValueStmtInitStack.back().Exprs.contains(E))
+    return nullptr;
+
+  return SingleValueStmtInitStack.back().Init;
+}
+
 void SILGenFunction::emitProfilerIncrement(ASTNode Node) {
   emitProfilerIncrement(ProfileCounterRef::node(Node));
 }