[NFC] let FunctionDecl::isReservedGlobalPlacementOperator return false when the funct...

[NFC] let FunctionDecl::isReservedGlobalPlacementOperator return false when the function decl is not allocation functions

Currently `FunctionDecl::isReservedGlobalPlacementOperator` will crash
if the function is not an allocation/deallocation function, which is
surprising. Also, its semantics is not consistent with
isReplaceableGlobalAllocationFunction, which will return false if the
function is not an allocation/deallocation function.

This patch make FunctionDecl::isReservedGlobalPlacementOperator not
crash if the function is not an allocation/deallocation function, which
is consistent with isReplaceableGlobalAllocationFunction too.