[PCH] Support writing BuiltinBitCastExprs to PCHs

eee944e7 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