Fix GCC5 build, require explicit this->... in this call inside a lambda (NFC)

Error was:

mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp:2247:36: error: cannot call member function 'mlir::LLVM::FastmathFlags mlir::LLVM::FMFAttr::getFlags() const' without object
     return bitEnumContains(getFlags(), flag);
                                    ^