[ast] Put the Stmt hierarchy on a diet for 64 bit targets.

Previously we would waste 32 bits on alignment, use LLVM_ALIGNAS to
free that space for derived classes an place. Sadly still have to #ifdef
out MSVC 2013 because it can't align based on a sizeof expr.

No intended functionality change. New byte counts:
sizeof(before)			|	sizeof(after)

LabelStmt: 32			|	LabelStmt: 24
SwitchStmt: 48			|	SwitchStmt: 40
WhileStmt: 40			|	WhileStmt: 32
DoStmt: 40			|	DoStmt: 32
ForStmt: 64			|	ForStmt: 56
ContinueStmt: 16		|	ContinueStmt: 8
BreakStmt: 16			|	BreakStmt: 8
ReturnStmt: 32			|	ReturnStmt: 24
AsmStmt: 40			|	AsmStmt: 32
GCCAsmStmt: 80			|	GCCAsmStmt: 72
MSAsmStmt: 96			|	MSAsmStmt: 88
SEHExceptStmt: 32		|	SEHExceptStmt: 24
SEHFinallyStmt: 24		|	SEHFinallyStmt: 16
SEHLeaveStmt: 16		|	SEHLeaveStmt: 8
CapturedStmt: 32		|	CapturedStmt: 24
CXXCatchStmt: 32		|	CXXCatchStmt: 24
CXXForRangeStmt: 72		|	CXXForRangeStmt: 64
ObjCAtFinallyStmt: 24		|	ObjCAtFinallyStmt: 16
ObjCAtSynchronizedStmt: 32	|	ObjCAtSynchronizedStmt: 24
ObjCAtThrowStmt: 24		|	ObjCAtThrowStmt: 16
ObjCAutoreleasePoolStmt: 24	|	ObjCAutoreleasePoolStmt: 16
llvm-svn: 233921