backport copy elision changes

This commit contains everything from the copy-elision-2
branch that does not have to do with copy elision directly,
but is generally useful for master branch.

 * All const values know their parents, when applicable, not
   just structs and unions.
 * Null pointers in const values are represented explicitly,
   rather than as a HardCodedAddr value of 0.
 * Rename "maybe" to "optional" in various code locations.
 * Separate DeclVarSrc and DeclVarGen
 * Separate PtrCastSrc and PtrCastGen
 * Separate CmpxchgSrc and CmpxchgGen
 * Represent optional error set as an integer, using the 0 value.
   In a const value, it uses nullptr.
 * Introduce type_has_one_possible_value and use it where applicable.
 * Fix debug builds not setting memory to 0xaa when storing
   undefined.
 * Separate the type of a variable from the const value of a variable.
 * Use copy_const_val where appropriate.
 * Rearrange structs to pack data more efficiently.
 * Move test/cases/* to test/behavior/*
 * Use `std.debug.assertOrPanic` in behavior tests instead of
   `std.debug.assert`.
 * Fix outdated slice syntax in docs.

test/compile_errors.zig

@@ -3220,7 +3220,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\    return 2;
         \\}
     ,
-        ".tmp_source.zig:2:15: error: unable to infer expression type",
+        ".tmp_source.zig:2:15: error: values of type 'comptime_int' must be comptime known",
     );
 
     cases.add(
@@ -3566,7 +3566,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\
         \\export fn entry() usize { return @sizeOf(@typeOf(a)); }
     ,
-        ".tmp_source.zig:2:11: error: expected type, found 'i32'",
+        ".tmp_source.zig:2:11: error: expected type 'type', found 'i32'",
     );
 
     cases.add(