Enhanced version of dip1036 implementation (#15715)

This implements the Enhanced Interpolated Expression Sequence proposal:

i"" or iq{} or q`` with a $(expression) in the middle are converted to a tuple of druntime types for future processing by library code.

compiler/test/runnable/interpolatedexpressionsequence.d

@@ -0,0 +1,51 @@
+import core.interpolation;
+
+alias AliasSeq(T...) = T;
+
+string simpleToString(T...)(T thing) {
+    string s;
+    foreach(item; thing)
+        // all the items provided by core.interpolation have
+        // toString to return an appropriate value
+        //
+        // then this particular example only has embedded strings
+        // and chars, to we can append them directly
+        static if(__traits(hasMember, item, "toString"))
+            s ~= item.toString();
+        else
+            s ~= item;
+
+    return s;
+}
+
+void main() {
+	int a = 1;
+	string b = "one";
+	// parser won't permit alias = i".." directly; i"..." is meant to
+	// be used as a function/template parameter at this time.
+	alias expr = AliasSeq!i"$(a) $(b)";
+	// elements from the source code are available at compile time, so
+	// we static assert those, but the values, of course, are different
+	static assert(expr[0] == InterpolationHeader());
+	static assert(expr[1] == InterpolatedExpression!"a"());
+	assert(expr[2] == a); // actual value not available at compile time
+	static assert(expr[3] == InterpolatedLiteral!" "());
+	// the parens around the expression are not included
+	static assert(expr[4] == InterpolatedExpression!"b"());
+	assert(expr[5] == b); // actual value not available at compile time
+	static assert(expr[6] == InterpolationFooter());
+
+	// it does currently allow `auto` to be used, it creates a value tuple
+	// you can embed any D expressions inside the parenthesis, and the
+	// token is not ended until you get the *outer* ) and ".
+	auto thing = i"$(b) $("$" ~ ')' ~ `"`)";
+	assert(simpleToString(thing) == "one $)\"");
+
+        assert(simpleToString(i"$b") == "$b"); // support for $ident removed by popular demand
+
+        // i`` and iq{} should also work
+        assert(simpleToString(i` $(b) is $(b)!`) == " one is one!");
+        assert(simpleToString(iq{ $(b) is $(b)!}) == " one is one!");
+        assert(simpleToString(i`\$('$')`) == "\\$"); // no \ escape there
+        assert(simpleToString(iq{{$('$')}}) == "{$}"); // {} needs to work
+}