MdeModulePkg/Variable: Fix volatile variable RT cache update logic

MdeModulePkg/Variable: Fix volatile variable RT cache update logic

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2333

During a SetVariable () invocation, UpdateVariable () is called.
UpdateVariable () contains logic to determine whether a volatile or
non-volatile UEFI variable was set so the corresponding runtime
cache can be updated to reflect the change. The current logic simply
evaluates Variable->Volatile to determine which runtime cache should
be updated.

The problem is Variable->Volatile does not always reflect whether a
volatile variable is being set. Variable->Volatile is set to TRUE
only in the case a pre-existing variable is found in the volatile
variable store. Therefore, the value is FALSE when a new volatile
variable is written.

This change updates the logic to take this into account. If a new
variable is written successfully, the Attributes will accurately
reflect whether the variable is non-volatile. If a pre-existing
variable is modified, the Volatile field will reflect the type of
variable (Attributes are not reliable; e.g. 0x0 indicates deletion).

* Observable symptom: A volatile variable that was set successfully
  might return EFI_NOT_FOUND when the variable should be found.

* The issue is a regression introduced to the variable services only
  when the variable runtime cache is enabled by the following PCD
  being set to TRUE:
  gEfiMdeModulePkgTokenSpaceGuid.PcdEnableVariableRuntimeCache

* The issue was implemented in commit aab3b9b9a1 but the PCD was not
  set to TRUE by default enabling the issue until commit e07b7d024a.

Fixes: aab3b9b9a1e5e1f3fa966fb1667fc3e6c47e7706
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>