MdePkg/BaseLib: Base64Decode(): don't declare variables in nested blocks

The EDK II C Coding Standards Specification (v2.20) strongly discourages
variable declarations in nested block scope:

5       Source Files
5.4     Code File Structure
5.4.1   Scoping Rules
5.4.1.1 Scope

> Block (local) Scope
>
> [...]
>
> Data declarations may follow the opening brace of a compound statement,
> regardless of nesting depth, and before any code generating statements
> have been entered. Other than at the outermost block of a function body,
> this type of declaration is strongly discouraged.

Hoist such variable declarations in Base64Decode() to the outermost
function scope.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Fixes: 35e242b698
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1980
Reported-by: Zhichao Gao <zhichao.gao@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Rebecca Cran <rebecca@bsdio.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daude <philmd@redhat.com>

MdePkg/Library/BaseLib/String.c

@@ -1978,6 +1978,9 @@ Base64Decode (
   UINT32  Accumulator;
   UINTN   OriginalDestinationSize;
   UINTN   SourceIndex;
+  CHAR8   SourceChar;
+  UINT32  Base64Value;
+  UINT8   DestinationOctet;
 
   if (DestinationSize == NULL) {
     return RETURN_INVALID_PARAMETER;
@@ -2054,10 +2057,6 @@ Base64Decode (
   // Decoding loop.
   //
   for (SourceIndex = 0; SourceIndex < SourceSize; SourceIndex++) {
-    CHAR8  SourceChar;
-    UINT32 Base64Value;
-    UINT8  DestinationOctet;
-
     SourceChar = Source[SourceIndex];
 
     //