fix Issue 11730 - associative array with Nullable!SysTime values: Called `get' on null Nullable!SysTime

When I fixed bug 6178, I internally used `cast(void)exp)` to suppress unexpected "has no side effect" error.

But, `CastExp` always try to resolve alias this of its operand to find most matching type to `void` via `Expression::castTo`. It should be disabled.
Merge pull request #2961 from 9rnsr/fix11730

[REG2.064] Issue 11730 - associative array with Nullable!SysTime values: Called `get' on null Nullable!SysTime
Remove dependency on Phobos from issue 10727 test case
fix Issue 11711 - Add __traits(getAliasThis)
switch case fallthrough is an error, but only enabled with -w
Merge pull request #2965 from rainers/issue11749

fix  Issue 11749 - switch case fallthrough error is enabled with -w, but cannot be made informational warning
Use ternary instead of pointer arithmetic for C++ mangling prefix
fix Issue 11748 - [ICE] function call as alias parameter of template gives ICE
Store RootObjects, not void*
Merge pull request #2969 from yebblies/mangleobj

[DDMD] Store RootObjects, not void*
Merge pull request #2966 from 9rnsr/fix11748

Issue 11748 - [ICE] function call as alias parameter of template gives ICE
Merge pull request #2939 from 9rnsr/fix11711

Issue 11711 - Add __traits(getAliasThis)
Merge pull request #2968 from yebblies/mangleptrarith

[DDMD] Use ternary instead of pointer arithmetic for C++ mangling prefix