[OpenMP] Fix OMPClauseReader::readClause() uninitialized variable warning. NFCI.

Fixes static analyzer uninitialized variable warning for the OMPClause - the function appears to cover all cases, but I've added an assertion to make sure.

llvm-svn: 371934
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 10b2a5c..fcb437e 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -12202,7 +12202,7 @@
 ////===----------------------------------------------------------------------===//
 
 OMPClause *OMPClauseReader::readClause() {
-  OMPClause *C;
+  OMPClause *C = nullptr;
   switch (Record.readInt()) {
   case OMPC_if:
     C = new (Context) OMPIfClause();
@@ -12403,6 +12403,8 @@
     C = OMPAllocateClause::CreateEmpty(Context, Record.readInt());
     break;
   }
+  assert(C && "Unknown OMPClause type");
+
   Visit(C);
   C->setLocStart(Record.readSourceLocation());
   C->setLocEnd(Record.readSourceLocation());