[ObjC] Add NSValue support for objc_boxed_expressions

Patch extends ObjCBoxedExpr to accept records (structs and unions):

typedef struct __attribute__((objc_boxable)) _Color {
  int r, g, b;
} Color;

Color color;
NSValue *boxedColor = @(color); // [NSValue valueWithBytes:&color objCType:@encode(Color)];

llvm-svn: 240761