[WebAssembly] Add missing default cases in switch statements

unsigned variable 'IntNo' has been declared but not been defined inside function
EmitWebAssemblyBuiltinExpr().

static code analysis tool complains about uninitialized variable "IntNo" since
this enters to default branch without setting any intrinsics and calls Function
*Callee = CGM.getIntrinsic(IntNo).

This patch fixes the problem by adding default cases in switch statements.