[PCH] Support writing BuiltinBitCastExprs to PCHs

eee944e7f adds the new BuiltinBitCastExpr, but does not set the Code member of
ASTStmtWriter. This is not correct and causes an assertion failue in
ASTStmtWriter::emit() when building PCHs that contain __builtin_bit_cast.  This
commit adds serialization::EXPR_BUILTIN_BIT_CAST and handles
ASTStmtWriter::Code properly.

Differential revision: https://reviews.llvm.org/D80360
diff --git a/clang/lib/Serialization/ASTWriterStmt.cpp b/clang/lib/Serialization/ASTWriterStmt.cpp
index 5e445b6..45cd54f 100644
--- a/clang/lib/Serialization/ASTWriterStmt.cpp
+++ b/clang/lib/Serialization/ASTWriterStmt.cpp
@@ -1655,6 +1655,7 @@
   VisitExplicitCastExpr(E);
   Record.AddSourceLocation(E->getBeginLoc());
   Record.AddSourceLocation(E->getEndLoc());
+  Code = serialization::EXPR_BUILTIN_BIT_CAST;
 }
 
 void ASTStmtWriter::VisitUserDefinedLiteral(UserDefinedLiteral *E) {