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
hyd-dev