[NFC] Initialize a variable to make buildbot green.

In r268085 "[MS] Make #pragma pack use PragmaStack<> class." there was an
uninitialized variable 'Alignment', which caused the following failure:
  http://lab.llvm.org:8011/builders/sanitizer-ppc64be-linux/builds/1758
Zero-initialize the variable to fix this failure.

llvm-svn: 268129

clang/lib/Sema/SemaAttr.cpp

@@ -78,7 +78,7 @@ void Sema::AddMsStructLayoutForRecord(RecordDecl *RD) {
 void Sema::ActOnPragmaOptionsAlign(PragmaOptionsAlignKind Kind,
                                    SourceLocation PragmaLoc) {
   PragmaMsStackAction Action = Sema::PSK_Reset;
-  unsigned Alignment;
+  unsigned Alignment = 0;
   switch (Kind) {
     // For all targets we support native and natural are the same.
     //