[flang] Support latest LLVM head with FIR - MLIR SideEffects interface change

Include new .td after LLVM changes:
0ddba0bd59

Tested to work with LLVM head ecd3e678bb
from 2020-03-13 01:45 am PCT

Backwards compatible with previous known compatible heads at least back to
fde9d33f71 from 2020-03-10, so need to
update LLVM builds if they previously work with FIR.

Original-commit: flang-compiler/f18@45454587c9
Reviewed-on: https://github.com/flang-compiler/f18/pull/1071

flang/include/flang/Optimizer/Dialect/FIROps.td

@@ -15,6 +15,7 @@
 #define FIR_DIALECT_FIR_OPS
 
 include "mlir/Interfaces/ControlFlowInterfaces.td"
+include "mlir/Interfaces/SideEffects.td"
 
 def fir_Dialect : Dialect {
   let name = "fir";